Systems and methods for performing pacing using multiple leadless pacemakers

ABSTRACT

An implantable system includes a first leadless pacemaker (LP1) implanted in or on a first chamber of a heart and a second leadless pacemaker (LP2) implanted in or on a second chamber of the heart. The LP1 is configured to time delivery of one or more pacing pulses delivered to the first chamber of the heart based on timing of cardiac activity associated with the second chamber of the heart detected by the LP1 itself. The LP1 is also configured to transmit implant-to-implant (i2i) messages to the LP2. The LP2 is configured to time delivery of one or more pacing pulses delivered to the second chamber of the heart based on timing of cardiac activity associated with the second chamber of the heart as determined based on one or more i2i messages received by the LP2 from the LP1.

FIELD OF TECHNOLOGY

Embodiments described herein generally relate to methods and systems forperforming various types of pacing using leadless pacemakers, as well asvarious leadless pacemaker embodiments and methods for use therewith.

BACKGROUND

Some cardiac pacing systems include one or more leadless pacemakers(LPs). Such an LP of a cardiac pacing system can be used to deliverpacing pulses to a cardiac chamber within (or on) which the LP isimplanted. In order to know when to deliver its pacing pulses, an LP mayneed to determine cardiac activity associated with another cardiacchamber. For an example, in order for an LP implanted in (or on) theright ventricular (RV) chamber to know when to deliver pacing pulses tothe RV chamber, the LP may need to determine cardiac activity associatewith the right atrial (RA) chamber, e.g., in order to achieve a desiredatrio-ventricular (AV) delay. For another example, in order for an LPimplanted in (or on) the RA chamber to know when to deliver pacingpulses to the RA chamber, the LP may need to determine cardiac activityassociate with the RV chamber, e.g., in order to achieve a desired VAinterval.

There are various different ways that an LP can determine cardiacactivity associated with another cardiac chamber, in order to known whento delivers its pacing pulses. For example, an LP implanted within or ona cardiac chamber can perform implant-to-implant (i2i) communicationwith another LP that is implanted within or on another cardiac chamber,or the LP can sense a far-field signal indicative of cardiac activity inanother chamber. Such i2i communication can involve one LP implanted ina cardiac chamber informing another LP implanted in another cardiacchamber of a paced or sensed event, so that coordinated synchronouspacing can be performed in multiple cardiac chambers. While suchtechniques are generally known, it would be beneficial to improve suchtechniques, e.g., to provide for improved pacing, improved i2icommunication, and/or improved far-field sensing. Further, it would bebeneficial to provide new and improved LPs that can be used to performsuch techniques.

SUMMARY

Certain embodiments of the present technology are related to implantablesystems and methods for use therewith. Such an implantable system caninclude a first leadless pacemaker (LP1) configured to be implanted inor on a first chamber of a heart, and a second leadless pacemaker (LP2)configured to be implanted in or on a second chamber of the heart. TheLP1 is configured to time delivery of one or more pacing pulsesdelivered to the first chamber of the heart based on timing of cardiacactivity associated with the second chamber of the heart detected by theLP1 itself. The LP1 is also configured to transmit implant-to-implant(i2i) messages to the LP2. The LP2 is configured to time delivery of oneor more pacing pulses delivered to the second chamber of the heart basedon timing of cardiac activity associated with the second chamber of theheart as determined based on one or more i2i messages received by theLP2 from the LP1. In certain embodiments, the LP1 is configured to beimplanted in or on a right ventricular (RV) chamber, and the LP2 isconfigured to be implanted in or on a right atrial (RA) chamber. Inother embodiments, the LP1 is configured to be implanted in or on a RAchamber, and the LP2 is configured to be implanted in or on a RVchamber.

In accordance with certain embodiments, the LP1 is configured to obtaina far-field signal from which electrical cardiac activity associatedwith the second chamber of the heart may be detected, and use thefar-field signal to thereby time delivery of one or more pacing pulsesdelivered to the first chamber of the heart, based on timing of cardiacactivity associated with the second chamber of the heart detected by theLP1 itself from the far-field signal. In certain such embodiments, theLP1 is configured to be implanted in or on the RV chamber, and the LP2is configured to be implanted in or on the RA chamber. Alternatively,the LP1 is configured to be implanted in or on the RA chamber, and theLP2 is configured to be implanted in or on the RV chamber.

In accordance with certain embodiments, the LP1 is configured to obtaina sensor signal from which mechanical cardiac activity associated withthe second chamber of the heart may be detected, and use the sensorsignal to thereby time delivery of one or more pacing pulses deliveredto the first chamber of the heart, based on timing of cardiac activityassociated with the second chamber of the heart detected by the LP1itself from the sensor signal. In certain such embodiments, the LP1 isconfigured to be implanted in or on the RV chamber, and the LP2 isconfigured to be implanted in or on the RA chamber. Alternatively, theLP1 is configured to be implanted in or on the RA chamber, and the LP2is configured to be implanted in or on the RV chamber.

In accordance with certain embodiments, the LP1 is configured to beimplanted in or on a RA chamber and the LP2 is configured to beimplanted in or on a RV chamber. The LP1 is configured to perform atleast one of AAI, ADI or ADD pacing, the LP2 is configured to perform atleast one of VVI, VDI or VDD pacing, and the LP1 and the LP2collectively provide DDD or DDI pacing or some other dual chamber pacingmode that provides synchronization between the LP1 and the LP2.

In accordance with other embodiments, the LP1 is configured to beimplanted in or on a RV chamber, and the LP2 is configured to beimplanted in or on a RA chamber. The LP1 is configured to perform atleast one of WI, VDI or VDD pacing, the LP2 is configured to perform atleast one of AAI, ADI or ADD pacing, and the LP1 and the LP2collectively provide DDD or DDI pacing or some other dual chamber pacingmode that provides synchronization between the LP1 and the LP2.

Certain embodiments of the present technology are related to methods foruse with an implantable system including a first leadless pacemaker(LP1) configured to be implanted in or on a first chamber of a heart,and a second leadless pacemaker (LP2) configured to be implanted in oron a second chamber of the heart. Such a method can include the LP1timing delivery of one or more pacing pulses delivered to the firstchamber of the heart based on timing of cardiac activity associated withthe second chamber of the heart detected by the LP1 itself, and the LP1transmitting implant-to-implant (i2i) messages to the LP2. Such a methodcan further include the LP2 timing delivery of one or more pacing pulsesdelivered to the second chamber of the heart based on timing of cardiacactivity associated with the second chamber of the heart as determinedbased on one or more i2i messages received by the LP2 from the LP1.

In accordance with certain embodiments, wherein the LP1 is configured tobe implanted in or on a RV chamber and the LP2 is configured to beimplanted in or on a RA chamber, the LP1 timing delivery of one or morepacing pulses delivered to the first chamber of the heart comprises theLP1 timing delivery of one or more pacing pulses delivered to the RVchamber based on timing of cardiac activity associated with the RAchamber detected by the LP1 itself. In such embodiments, the LP2 timingdelivery of one or more pacing pulses delivered to the second chamber ofthe heart comprises the LP2 timing delivery of one or more pacing pulsesdelivered to the RA chamber based on timing of cardiac activityassociated with the RV chamber as determined based on one or more i2imessages received by the LP2 from the LP1.

In accordance with certain embodiments, wherein the LP1 is configured tobe implanted in or on a RA chamber and the LP2 is configured to beimplanted in or on a RV chamber, the LP1 timing delivery of one or morepacing pulses delivered to the first chamber of the heart comprises theLP1 timing delivery of one or more pacing pulses delivered to the RAchamber based on timing of cardiac activity associated with the RVchamber detected by the LP1 itself. In such embodiments, the LP2 timingdelivery of one or more pacing pulses delivered to the second chamber ofthe heart comprises the LP2 timing delivery of one or more pacing pulsesdelivered to the RV chamber based on timing of cardiac activityassociated with the RA chamber as determined based on one or more i2imessages received by the LP2 from the LP1.

In accordance with certain embodiments, the LP1 timing delivery of oneor more pacing pulses delivered to the first chamber of the heartcomprises the LP1 obtaining a far-field signal from which electricalcardiac activity associated with the second chamber of the heart may bedetected, and the LP1 using the far-field signal to thereby timedelivery of one or more pacing pulses delivered to the first chamber ofthe heart, based on timing of cardiac activity associated with thesecond chamber of the heart detected by the LP1 itself from thefar-field signal. In certain such embodiments, wherein the LP1 isconfigured to be implanted in or on the RV chamber and the LP2 isconfigured to be implanted in or on the RA chamber, the LP1 obtaining afar-field signal comprises the LP1 obtaining a far-field signal fromwhich electrical cardiac activity associated with the RA chamber may bedetected, and the LP1 using the far-field signal comprises the LP1 usingthe far-field signal to thereby time delivery of one or more pacingpulses delivered to the RV chamber, based on timing of cardiac activityassociated with the RA chamber detected by the LP1 itself from thefar-field signal. In alternative such embodiments, wherein the LP1 isconfigured to be implanted in or on the RA chamber and the LP2 isconfigured to be implanted in or on the RV chamber, the LP1 obtaining afar-field signal comprises the LP1 obtaining a far-field signal fromwhich electrical cardiac activity associated with the RV chamber may bedetected, and the LP1 using the far-field signal comprises the LP1 usingthe far-field signal to thereby time delivery of one or more pacingpulses delivered to the RA chamber, based on timing of cardiac activityassociated with the RV chamber detected by the LP1 itself from thefar-field signal.

In accordance with certain embodiments, the LP1 timing delivery of oneor more pacing pulses delivered to the first chamber of the heartcomprises the LP1 obtaining a sensor signal from which mechanicalcardiac activity associated with the second chamber of the heart may bedetected, and the LP1 using the sensor signal to thereby time deliveryof one or more pacing pulses delivered to the first chamber of theheart, based on timing of cardiac activity associated with the secondchamber of the heart detected by the LP1 itself from the sensor signal.In certain such embodiments, wherein the LP1 is configured to beimplanted in or on a RV chamber and the LP2 is configured to beimplanted in or on a RA chamber, the LP1 obtaining a sensor signalcomprises the LP1 obtaining a sensor signal from which mechanicalcardiac activity associated with the RA chamber may be detected, and theLP1 using the sensor signal comprises the LP1 using the sensor signal tothereby time delivery of one or more pacing pulses delivered to the RVchamber, based on timing of cardiac activity associated with the RAchamber detected by the LP1 itself from the sensor signal. Inalternative such embodiments, wherein the LP1 is configured to beimplanted in or on a RA chamber and the LP2 is configured to beimplanted in or on a RV chamber, the LP1 obtaining a sensor signalcomprises the LP1 obtaining a sensor signal from which mechanicalcardiac activity associated with the RV chamber may be detected, and theLP1 using the sensor signal comprises the LP1 using the sensor signal tothereby time delivery of one or more pacing pulses delivered to the RAchamber, based on timing of cardiac activity associated with the RVchamber detected by the LP1 itself from the sensor signal.

Certain embodiments of the present technology are directed to animplantable system that includes a first leadless pacemaker (LP1)configured to be implanted in or on a RA chamber, and a second leadlesspacemaker (LP2) configured to be implanted in or on a RV chamber,wherein the LP1 is configured to perform at least one of AAI, ADI or ADDpacing, the LP2 is configured to perform at least one of WI, VDI or VDDpacing, and the LP1 and the LP2 collectively provide DDD or DDI pacingor some other dual chamber pacing mode that provides synchronizationbetween the LP1 and the LP2. Where two LPs (e.g., the LP1 and the LP2)are said to be synchronized or have synchronization provided, this meansthat the pacing performed by at least one of the LPs is timed relativeto paced events delivered by and/or sensed events sensed by the otherone of the LPs. Accordingly, two LPs can be said to be synchronizedwhere there is VA synchrony but not AV synchrony, VA synchrony but notAV synchrony, or both VA and AV synchrony

This summary is not intended to be a complete description of theembodiments of the present technology. Other features and advantages ofthe embodiments of the present technology will appear from the followingdescription in which the preferred embodiments have been set forth indetail, in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present technology relating to both structure andmethod of operation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIG. 1A illustrates a system formed in accordance with certainembodiments described herein as implanted in a heart.

FIG. 1B is a block diagram of an exemplary leadless pacemaker (LP) inaccordance with certain embodiments herein.

FIG. 2 illustrates an LP in accordance with certain embodiments herein.

FIG. 3 is a timing diagram demonstrating one embodiment of implant toimplant (i2i) communication for a paced event.

FIG. 4 is a timing diagram demonstrating one embodiment of i2icommunication for a sensed event.

FIG. 5 is a diagram that is used to show how the orientation of twodifferent LPs can be quantified in accordance with certain embodimentsof the present technology.

FIG. 6 is a high level flow diagram that is used to describe embodimentsof the present technology where an LP implanted in or on a chamber of aheart preferably or by default attempts to time its pacing pulses(relative to activity of a remote chamber) based on electrical cardiacactivity associated with another chamber of the heart as determined froma sensed far-field signal, and as a backup, uses i2i messages receivedfrom an LP implanted in or on the other chamber when a far-field signalis not successfully detected.

FIG. 7 is a high level flow diagram that is used to describe embodimentsof the present technology where an LP implanted in or on a chamber of aheart preferably or by default attempts to time its pacing pulses(relative to activity of a remote chamber) based on mechanical cardiacactivity associated with another chamber of the heart as determined froma sensor signal, and as a backup, uses i2i messages received from an LPimplanted in or on the other chamber of the heart when a far-fieldsignal is not successfully detected.

FIG. 8 is a high level flow diagram that is used to describe embodimentsof the present technology where an LP implanted in or on a chamber of aheart preferably or by default uses i2i messages (received from an LPimplanted in or on another chamber of the heart) to time its pacing, andas a backup, uses a far-field signal and/or sensor signal to time itspacing when valid i2i messages are not successfully received.

FIG. 9A is an illustration of an LP according to an embodiment of thepresent technology.

FIG. 9B is an illustration of an LP according to another embodiment ofthe present technology.

FIG. 10A is a high level flow diagram that is used to summarize certainmethods of the present technology that can be used with the LPillustrated in FIG. 9A.

FIG. 10B is a high level flow diagram that is used to summarize certainmethods of the present technology that can be used with the LPillustrated in FIG. 9B.

FIG. 11 shows a block diagram of an embodiment of an LP that isimplanted into a patient as part of an implantable cardiac system inaccordance with certain embodiments herein.

DETAILED DESCRIPTION

Certain embodiments of the present technology relate to implantablesystems, and methods for use therewith, that can be used to performvarious pacing schemes using one or more LPs. For example, certainembodiments of the present technology relate to implantable systems, andmethods for use therewith, that can be used perform DDD pacing using twoLPs. More specifically, in accordance with certain embodiments of thepresent technology, an LP is implanted in (or on) a patient's RV chamberand is used to perform VDD pacing, and another LP is implanted in (oron) the patient's RA chamber and is used to perform ADD, AAI, or ADIpacing. Collectively, the two LPs are used to perform DDD or DDI pacingor some other dual chamber pacing mode that provides synchronizationbetween the LP1 and the LP2. Where two LPs (e.g., the LP1 and the LP2)are said to be synchronized or have synchronization provided, this meansthat the pacing performed by at least one of the LPs is timed relativeto paced events delivered by and/or sensed events sensed by the otherone of the LPs. Accordingly, two LPs can be said to be synchronizedwhere there is VA synchrony but not AV synchrony, VA synchrony but notAV synchrony, or both VA and AV synchrony

Any one of various different algorithms can be used to achieve such dualchamber pacing modes. When referring to various types of pacing schemesherein, three letters are often used to refer to the type of pacing. Inother words, a three position pacemaker code is often used, with thefollowing nomenclature followed: the first position refers to thecardiac chamber paced; the second position refers to the cardiac chambersensed; and the third position refers to the response to a sensed event.In the first and second positions, the letter O means none, the letter Ameans Atrium, the letter V means Ventricle, and the letter D means Dual(i.e., A and V). In the third position the letter O means none, theletter I means Inhibited, the letter T means Triggered (aka Tracked),and the letter D means Dual (i.e., T+I). The below Table 1 summarizesthis pacemaker nomenclature.

TABLE 1 Position 1 Position 2 Position 3 (Chamber Paced) (ChamberSensed) (Response to Sensed Event) O = none O = none O = none A = AtriumA = Atrium I = Inhibited V = Ventricle V = Ventricle T = Triggered (akaTracked) D = Dual (A + V) D = Dual (A + V) D = Dual (I + T)

Accordingly, if an LP in the patient's RV chamber performs VDD pacing,that means it paces only the RV chamber, senses both atrial andventricular activity, and inhibits pacing of the RV if a sensed event isdetected within a specified interval (the AV interval) or triggerspacing of the RV at the end of the specified interval (the AV interval)if a sensed event is not detected within that specified interval (the AVinterval). For another example, if an LP in the patient's RA chamberperforms AAI pacing, that means it paces only the RA chamber, sensesonly atrial activity, and inhibits pacing of the RA chamber if a sensedevent is detected within a specified interval. Where the second positionincludes a “D”, the LP will need to be aware of activity in its ownchamber and in another chamber in or one which the LP is not implanted.Activity in another chamber can be determined from a far-field signaland/or from an i2i message received from another LP that is in or onethe other chamber.

The LP in (or on) the patient's RA chamber can also be referred to asthe aLP, and the LP in (or on) the patient's RV chamber can also bereferred to as the vLP. There are various different ways that anexternal programmer can instruct an aLP and an vLP to perform certainpacing modes that are the equivalent of pacing modes performed usingconventional (i.e., non-leadless) pacemakers. For example, assume thatit is desired that an aLP performs one of ADD, AAI, or ADI pacing, andthe vLP performs VDD pacing, such that collectively the aLP and the vLPperform DDD or DDI pacing. The external programmer can instruct the aLPto perform a specific one of ADD, AAI, or ADI pacing, and externalprogrammer can instruct the vLP to perform VDD pacing. Alternatively,the external programmer can instruct both of the aLP and the vLP toperform DDD or DDI pacing, in response to which the aLP will know (basedon how it is programmed) to perform a specific one of ADD, AAI, or ADIpacing, and the vLP will know (based on how it is programmed) to performVDD pacing, such that collectively the aLP and the vLP will perform DDDor DDI pacing. Other variations are also possible.

When the LP in (or on) the RV chamber performs VDD pacing, it shouldknow when certain cardiac activity (e.g., atrial contractions) occur inthe RA chamber, so that it knows the appropriate times at which to pacethe RV chamber. In accordance with certain embodiments, the LP in (oron) the RV chamber senses a far-field signal from which electricalcardiac activity associated with the RA chamber may be detected, and theLP in (or on) the RV chamber times its delivery of RV pacing pulsesbased on the timing of the electrical cardiac activity associated withthe RA chamber detected from the far-field signal. For example, the LPin the RV chamber may be able to detect P waves from the far-fieldsignal it senses in order to know when to deliver RV pacing pulses. TheLP in the RV chamber can alternatively or additionally determine thetiming of atrial cardiac activity based on i2i messages received from anLP implanted in the RA chamber. As will be described in additionallydetail below, delivery of an i2i message from the LP in the RA chamber(to the LP in the RV chamber) can be via pulses generated by the LP inthe RA chamber in response to a sensed or paced atrial event, whereinsuch pulses can be generated (and thus delivered) prior to, during, orafter an atrial refractor period associated with the atrial event,depending upon implementation. The LP in the RV chamber canalternatively or additionally use a sensor (e.g., an accelerometer or apressure sensor) to produce a sensor signal from which the LP in the RVchamber can detect cardiac mechanical activity associated with the RAchamber, and time its RV pacing pulses based thereon. Combinations ofthese aforementioned embodiments are also described herein.

In certain embodiments, the LP in (or on) the RV chamber primarilyrelies on far-field sensing of electrical cardiac activity associatedwith the RA chamber to time delivery of RV pacing pulses, but uses i2imessaging as a backup. In alternative embodiments, the LP in (or on) theRV chamber primarily times delivery of RV pacing pulses based on thetiming of cardiac activity associated with the RA chamber as determinedfrom i2i messages, and uses far-field sensing as backup. Othervariations are also possible and within the scope of the embodimentsdescribed herein.

When the LP in (or on) the RA chamber performs ADD pacing, it shouldknow when certain cardiac activity (e.g., ventricular contractions)occur in the RV chamber, so that it knows the appropriate times at whichto pace the RA chamber. In accordance with certain embodiments, the LPin (or on) the RA chamber senses a far-field signal from whichelectrical cardiac activity associated with the RV chamber may bedetected, and the LP in (or on) the RA chamber times its delivery of RApacing pulses based on the timing of the electrical cardiac activityassociated with the RV chamber detected from the far-field signal. Forexample, the LP in the RA chamber may be able to detect R waves from thefar-field signal it senses in order to know when to deliver RA pacingpulses. The LP in the RA chamber can alternatively or additionallydetermine the timing of ventricular cardiac activity based on i2imessages received from an LP implanted in the RV chamber. The LP in theRA chamber can alternatively or additionally use a sensor (e.g., anaccelerometer or a pressure sensor) to produce a sensor signal fromwhich the LP in the RA chamber can detect cardiac mechanical activityassociated with the RV chamber, and time its RA pacing pulses basedthereon. Combinations of these aforementioned embodiments are alsodescribed herein.

In certain embodiments, the LP in (or on) the RA chamber primarilyrelies on far-field sensing of electrical cardiac activity associatedwith the RV chamber to time delivery of RA pacing pulses, but uses i2imessaging as a backup. In alternative embodiments, the LP in (or on) theRA chamber primarily times delivery of RA pacing pulses based on thetiming of cardiac activity associated with the RV chamber as determinedfrom i2i messages, and uses far-field sensing as backup. Othervariations are also possible and within the scope of the embodimentsdescribed herein.

In certain embodiments, the LP in (or on) the RV chamber performs WIpacing (e.g., using programmed VV intervals), and the LP in (or on) theRA chamber performs ADD or ADI pacing. The ADD or ADI pacing performedby the LP in (or on) the RA chamber can involve pacing and sensing inthe RA chamber, sensing in the RV chamber (achieved by sensing afar-field signal, or producing a sensor signal from which mechanicalcardiac activity in the RV chamber can be detected). Such a system wouldbe useful for patients having sinus rhythm with heart block andintermittent atrial arrhythmia. An advantage of this system is that itcould achieve dual chamber pacing and sensing with only one of LPsobtaining a far-field signal indicative of ventricular cardiac activity(which is much stronger than a far-field signal indicative of atrialcardiac activity).

In certain embodiments, the LP in (or on) the RV chamber performs VDIpacing (e.g., using programmed W and VA intervals), and the LP in (oron) the RA chamber performs AAI pacing. Such a system essentiallyprovides for DDI pacing.

Before providing addition details of the specific embodiments of thepresent technology mentioned above, as well as additional embodiments ofthe present technology, an exemplary system in which embodiments of thepresent technology can be used will first be described with reference toFIGS. 1A, 1B and 2. More specifically, FIGS. 1A, 1B and 2 will be usedto describe an exemplary cardiac pacing system, wherein pacing andsensing operations can be performed by multiple medical devices, whichmay include one or more LPs, an implantable cardioverter-defibrillator(ICD), such as a subcutaneous-ICD, and/or a programmer reliably andsafely coordinate pacing and/or sensing operations. Later on, specificembodiments of LPs according to certain embodiments of the presenttechnology will be described, e.g., with reference to FIGS. 9A and 9B.

FIG. 1A illustrates a system 100 formed in accordance with certainembodiments herein as implanted in a heart 101. The system 100 comprisestwo or more LPs 102 and 104 located in different chambers of the heart.LP 102 is located in the RA chamber, while LP 104 is located in the RVchamber. LPs 102 and 104 can communicate with one another to inform oneanother of various local physiologic activities, such as local intrinsicevents, local paced events and the like. LPs 102 and 104 may beconstructed in a similar manner, but operate differently based uponwhich chamber LP 102 or 104 is located. It is noted that the RA chamberis also known as the right atrium, and the acronym RA can be used torefer to the “right atrium” or to refer to the “right atrial” chamber.Similarly, the RV chamber is also known as the right ventricle, and theacronym RV can be used to refer to the “right ventricle” or to refer tothe “right ventricular” chamber. It is also noted that the terms“cardiac chamber”, “chamber of the heart”, and “chamber of a patient'sheart” are used interchangeably herein.

In accordance with certain embodiments, the LP 102 is used to performADD pacing, the LP 104 is used to perform VDD pacing, and the LPs 102and 104 are collectively used to perform DDD pacing. The ADD pacing(performed by the LP 102) involves atrial pacing, ventricular and atrial(i.e., dual) sensing, and dual (i.e., triggered and inhibited) responseto a sensed event. The VDD pacing (performed by the LP 104) involvesventricular pacing, atrial and ventricular (i.e., dual) sensing, anddual (i.e., triggered and inhibited) response to a sensed event. The DDDpacing (performed collectively by the LPs 102 and 104) involves atrialand ventricular (i.e., dual) pacing, atrial and ventricular (i.e., dual)sensing, and dual (i.e., triggered and inhibited) response to a sensedevent.

In some embodiments, LPs 102 and 104 communicate with one another, withan ICD 106, and with an external device (e.g., programmer) 109 throughwireless transceivers, communication coils and antenna, and/or byconductive communication through the same electrodes as (or one or moredifferent electrodes than) used for sensing and/or delivery of pacingtherapy. When conductive communication is performed using electrodes,the system 100 may omit an antenna or telemetry coil in one or more ofLPs 102 and 104.

In some embodiments, one or more LPs 102 and 104 can be co-implantedwith the ICD 106. Each LP 102, 104 uses two or more electrodes locatedwithin, on, or within a few centimeters of the housing of the LP, forpacing and sensing at the cardiac chamber, for bidirectionalcommunication with one another, with the programmer 109 (or some otherexternal device), and the ICD 106.

In FIG. 1A, the two LPs 102 and 104 are shown as being implantedendocardially, i.e., within respective cardiac chambers. In other words,in FIG. 1A each of the LPs 102 and 104 is shown as being implanted in arespective cardiac chamber, i.e., the LP 102 is shown as being implantedin the RA chamber, and the LP 104 is shown as being implanted in the RVchamber. Alternatively, one or both of the LPs 102 and 104 can beimplanted epicardially (on the external heart surface) by affixing tothe exterior surface of the heart. For example, it would also bepossible for the LP 102 to be affixed to an exterior surface of the RAchamber, in which case the LP 102 can be said to be implanted on (ratherthan in) the RA chamber. Similarly, it would also be possible for the LP104 to be affixed to an exterior of the RV chamber, in which case the LP104 can be said to be implanted on (rather than in) the RV chamber. Moregenerally, an LP can either be implanted in or on the cardiac chamberthat the LP is being used to pace. It is noted that the terms “implantedin,” “implanted within,” “located in,” and “located within” are usedinterchangeably herein when referring to where a particular LP isimplanted. Further, it is noted that the terms “located on” and“implanted on” are used interchangeably herein when referring to where aparticular LP is implanted. The cardiac chamber within or on which aparticular LP is implanted can be referred to as a “local chamber”,while another chamber (within or on which the particular LP is notimplanted) can be referred to as a “remote chamber”.

In accordance with certain embodiments, methods are provided forcoordinating operation between LPs located in or on different cardiacchambers of the heart. Some such methods can configure a local LP toreceive communications from a remote LP through conductivecommunication. Some such methods rely on a local LP sensing a far-fieldsignal and/or a sensor signal to itself monitor cardiac activityassociated with a remote cardiac chamber.

Referring to FIG. 1B, a block diagram shows exemplary electronics withinLPs 102 and 104. LP 102, 104 includes first and second receivers 120 and122 that collectively define separate first and second communicationchannels 105 and 107 (FIG. 1A), (among other things) between LPs 102 and104. Although first and second receivers 120 and 122 are depicted, inother embodiments, LP 102, 104 may only include first receiver 120, ormay include additional receivers other than first and second receivers120 and 122. As will be described in additional detail below, the pulsegenerator 116 can function as a transmitter that transmitsimplant-to-implant (i2i) communication signals using the electrodes 108.Usage of the electrodes 108 for communication enables the one or moreLPs 102 and 104 to perform antenna-less and telemetry coil-lesscommunication.

In accordance with certain embodiments, when one of the LPs 102 and 104senses an intrinsic event or delivers a paced event, the correspondingLP 102, 104 transmits an implant event message to the other LP 102, 104.For example, when an atrial LP 102 senses/paces an atrial event, theatrial LP 102 transmits an implant event message including an eventmarker indicative of a nature of the event (e.g., intrinsic/sensedatrial event, paced atrial event). When a ventricular LP 104senses/paces a ventricular event, the ventricular LP 104 transmits animplant event message including an event marker indicative of a natureof the event (e.g., intrinsic/sensed ventricular event, pacedventricular event). In certain embodiments, LP 102, 104 transmits animplant event message to the other LP 102, 104 preceding the actual pacepulse so that the remote LP can blank its sense inputs in anticipationof that remote pace pulse (to prevent inappropriate crosstalk sensing).

Still referring to FIG. 1B, each LP 102, 104 is shown as including acontroller 112 and a pulse generator 116. The controller 112 caninclude, e.g., a microprocessor (or equivalent control circuitry), RAMand/or ROM memory, logic and timing circuitry, state machine circuitry,and I/O circuitry, but is not limited thereto. The controller 112 canfurther include, e.g., timing control circuitry to control the timing ofthe stimulation pulses (e.g., pacing rate, atrio-ventricular (AV) delay,atrial interconduction (A-A) delay, or ventricular interconduction (V-V)delay, etc.). Such timing control circuitry may also be used for thetiming of refractory periods, blanking intervals, noise detectionwindows, evoked response windows, alert intervals, marker channeltiming, and so on. The controller 112 can further include otherdedicated circuitry and/or firmware/software components that assist inmonitoring various conditions of the patient's heart and managing pacingtherapies. The controller 112 and the pulse generator 116 may beconfigured to transmit event messages, via the electrodes 108, in amanner that does not inadvertently capture the heart in the chamberwhere LP 102, 104 is located, such as when the associated chamber is notin a refractory state. In addition, a LP 102, 104 that receives an eventmessage may enter an “event refractory” state (or event blanking state)following receipt of the event message. The event refractory/blankingstate may be set to extend for a determined period of time after receiptof an event message in order to avoid the receiving LP 102, 104 frominadvertently sensing another signal as an event message that mightotherwise cause retriggering. For example, the receiving LP 102, 104 maydetect a measurement pulse from another LP 102, 104 or programmer 109.

In accordance with certain embodiments herein, the programmer 109 maycommunicate over a programmer-to-LP channel, with LP 102, 104 utilizingthe same communication scheme. The external programmer 109 may listen tothe event message transmitted between LP 102, 104 and synchronizeprogrammer to implant communication such that programmer 109 does nottransmit communication signals 113 until after an implant to implantmessaging sequence is completed. Alternatively, the external programmer109 may wait for a directed communication message transmitted to theexternal programmer 109 from LP 102 or 104 that indicates to theexternal programmer 109 that that the LP is ready to trade communicationsignals 113 with the external programmer 109. An LP 102, 104 can alsocommunicate with other types of external devices besides the externalprogrammer 109, such as, but not limited to, an external monitor.

In accordance with certain embodiments, LP 102, 104 may combine transmitoperations with therapy. The transmit event marker may be configured tohave similar characteristics in amplitude and pulse-width to a pacingpulse and LP 102, 104 may use the energy in the event messages to helpcapture the heart. For example, a pacing pulse may normally be deliveredwith pacing parameters of 2.5V amplitude, 500 ohm impedance, 60 bpmpacing rate, 0.4 ms pulse-width. The foregoing pacing parameterscorrespond to a current draw of about 1.9 μA. The same LP 102, 104 mayimplement an event message utilizing event signaling parameters foramplitude, pulse-width, pulse rate, etc. that correspond to a currentdraw of approximately 0.5 μA for transmit.

LP 102, 104 may combine the event message transmissions with pacingpulses. For example, LP 102, 104 may use a 50 μs wakeup transmit pulsehaving an amplitude of 2.5V which would draw 250 nC (nano Coulombs) foran electrode load of 500 ohm. The pulses of the transmit event messagemay be followed by an event message encoded with a sequence of shortduration pulses (for example 16, 2 μs on/off bits) which would draw anadditional 80 nC. The event message pulse would then be followed by theremaining pulse-width needed to reach an equivalent charge of a nominal0.4 ms pace pulse. In this case, the current necessary to transmit themarker is essentially free as it was used to achieve the necessary pacecapture anyhow. With this method, the savings in transmit current couldbe budgeted for the receiver or would allow for additional longevity.When LP 102 or 104 senses an intrinsic event, it can send aqualitatively similar event pulse sequence (but indicative of a sensedevent) without adding the pace pulse remainder. As LP 102, 104 longevitycalculations are designed based on the assumption that LP 102, 104 willdeliver pacing therapy 100% of the time, transmitting an intrinsic eventmarker to another LP 102, 104 will not impact the nominal calculated LPlongevity.

In some embodiments, the individual LP 102 can comprise a hermetichousing 110 configured for placement on or attachment to the inside oroutside of a cardiac chamber and at least two leadless electrodes 108proximal to the housing 110 and configured for bidirectionalcommunication with at least one other device 106 within or outside thebody. As will be described in additional detail below, with reference toFIGS. 9A and 9B, in certain embodiments an individual LP includes twohermetic housings, one of which includes electronic circuitry, and theother of which includes a battery.

Referring to FIG. 1B, the LP 102 (or 104) is shown as including anaccelerometer 154 which can be hermetically contained within the housing110. The accelerometer 154 can be any one of various different types ofwell known accelerometers, or can be a future developed accelerometer.For one example, the accelerometer 154 can be or include, e.g., a MEMS(micro-electromechanical system) multi-axis accelerometer of the typeexploiting capacitive or optical cantilever beam techniques, or apiezoelectric accelerometer that employs the piezoelectric effect ofcertain materials to measure dynamic changes in mechanical variables.Where the accelerometer is a multi-axis accelerometer it can include twoor three sensors aligned along orthogonal axes. Exemplary multi-axisaccelerometers (also referred to as multi-dimensional accelerometers)that can be used are described in U.S. Pat. No. 6,658,292 (Kroll et al.)and U.S. Pat. No. 6,466,821 (Pianca et al.), each of which isincorporated herein by reference. For another example, a commerciallyavailable micro-electromechanical system (MEMS) accelerometer marketedas the ADXL345 by Analog Devices, Inc. (headquartered in Norwood, Mass.)is a three-axis accelerometer and includes polysilicon springs thatprovide a resistance against acceleration forces. The term MEMS has beendefined generally as a system or device having micro-circuitry on a tinysilicon chip into which some mechanical device such as a mirror or asensor has been manufactured. The aforementioned ADXL345 includes amicro-machined accelerometer co-packaged with a signal processing IC.

Another commercially available MEMS accelerometer is the ADXL327 byAnalog Devices, Inc., which is a small, thin, low power, complete threeaxis accelerometer with signal conditioned voltage outputs. In theADXL327, the mechanical sensor and signal conditioning IC are packagedtogether. A further commercially available MEMS accelerometer that canbe used is the LIS3DH three-axis accelerometer by STMicroelectronics(headquartered in Geneva, Switzerland). Additional and/or alternativetypes of accelerometers may also be used. For example, it is also withinthe scope of the present technology for the accelerometer 154 to be abeam-type of accelerometer, an example of which is described in U.S.Pat. No. 6,252,335 (Nilsson et al.), which is incorporated herein byreference.

The accelerometer 154 can be, e.g., a one-dimensional (1D) accelerometer(also known as a one-axis accelerometer), a two-dimensional (2D)accelerometer (also known as a two-axis accelerometer), or athree-dimensional (3D) accelerometer (also known as a three-axisaccelerometer). A 1D accelerometer measures acceleration along one axis,e.g., the z-axis. A 2D accelerometer measures acceleration along twoaxes that are orthogonal to one another, e.g., the z-axis, and the x- ory-axis. A 3D accelerometer measures acceleration along three axes thatare orthogonal to one another, e.g., the z-axis, the x-axis, and they-axis. Each measure of acceleration (i.e., rate of change of velocity)can actually be a measure of proper acceleration, which is the rate ofchange of velocity of a body in its own instantaneous rest frame. Forexample, an accelerometer at rest on the surface of the Earth willmeasure an acceleration due to Earth's gravity, straight upwards (bydefinition) of g≈9.81 m/s{circumflex over ( )}2.

Where an LP (e.g., LP 102 or 104) includes an accelerometer within ahousing of the LP or attached thereto, the accelerometer can be used tomeasure the acceleration of the LP along one or more axes, whichmeasurement(s) can be used to determine the orientation of the LP.Accordingly, because the output(s) of the accelerometer can be used todetermine the orientation of the LP, it can be said that the output(s)of the accelerometer (e.g., 154) are indicative of an orientation of theLP (e.g., LP 102 or 104). More specifically, in accordance with certainembodiments, the controller 112 of an LP 102 (or 104) receives one ormore outputs output(s) of the accelerometer 154, which is/are indicativeof an orientation of the LP 102 (or 104). In such embodiments, thecontroller 112 can determine, based on the output(s) received from theaccelerometer 154, an actual orientation of the LP 102 (or 104). Eachoutput of the accelerometer 154 can comprise a respective signal.

One or more signals produced and output by the accelerometer 154 may beanalyzed with respect to frequency content, energy, duration, amplitudeand/or other characteristics. Such signals may or may not be amplifiedand/or filtered prior to being analyzed. For example, filtering may beperformed using lowpass, highpass and/or bandpass filters. The signalsoutput by the accelerometer 154 can be analog signals, which can beanalyzed in the analog domain, or can be converted to digital signals(by an analog-to-digital converter) and analyzed in the digital domain.Alternatively, the signals output by the accelerometer 154 can alreadybe in the digital domain.

The one or more signals output by the accelerometer 154 can be analyzedby the controller 112 and/or other circuitry. In certain embodiments,the accelerometer 154 is packaged along with an integrated circuit (IC)that is designed to analyze the signal(s) it generates. In suchembodiments, one or more outputs of the packaged sensor/IC can be anindication of acceleration along one or more axes. In other embodiments,the accelerometer 154 can be packaged along with an IC that performssignal conditioning (e.g., amplification and/or filtering), performsanalog-to-digital conversions, and stores digital data (indicative ofthe sensor output) in memory (e.g., RAM, which may or may not be withinthe same package). In such embodiments, the controller 112 or othercircuitry can read the digital data from the memory and analyze thedigital data. Other variations are also possible, and within the scopeof embodiments of the present technology. In accordance with certainembodiments of the present technology, described in additional detailbelow, a sensor signal produced by the accelerometer 154 of an LPimplanted in or on a cardiac chamber can be used to detect mechanicalcardiac activity associated with another cardiac chamber.

FIG. 1B depicts a single LP 102 (or 104) and shows the LP's functionalelements substantially enclosed in a hermetic housing 110. The LP 102(or 104) has at least two electrodes 108 located within, on, or near thehousing 110, for delivering pacing pulses to and sensing electricalactivity from the muscle of the cardiac chamber, and for bidirectionalcommunication with at least one other device within or outside the body.Hermetic feedthroughs 130, 131 conduct electrode signals through thehousing 110. The housing 110 contains a primary battery 114 to supplypower for pacing, sensing, and communication. The housing 110 alsocontains circuits 132 for sensing cardiac activity from the electrodes108, receivers 120, 122 for receiving information from at least oneother device via the electrodes 108, and the pulse generator 116 forgenerating pacing pulses for delivery via the electrodes 108 and alsofor transmitting information to at least one other device via theelectrodes 108. The housing 110 can further contain circuits formonitoring device health, for example a battery current monitor 136 anda battery voltage monitor 138, and can contain circuits for controllingoperations in a predetermined manner.

In FIG. 1B, all of the components shown within the housing 110, besidesthe battery 114, can be referred generally as electrical circuitry orelectronics of the LP 102, 104. In FIG. 1B the battery 114 and theelectronics are shown as being within the same housing 110. In certainembodiments of the present technology, described below with reference toFIGS. 9A and 9B, the battery 114 and the electronics are included withinseparate respective electrically conductive housings (e.g., 912 and 922in FIG. 9A) that are electrically isolated from one another.

The electrodes 108 can be configured to communicate bidirectionallyamong the multiple LPs and/or the implanted ICD 106 to coordinate pacingpulse delivery and optionally other therapeutic or diagnostic featuresusing messages that identify an event at an individual LP originatingthe message and an LP receiving the message react as directed by themessage depending on the origin of the message. An LP 102, 104 thatreceives the event message reacts as directed by the event messagedepending on the message origin or location. In some embodiments orconditions, the two or more leadless electrodes 108 can be configured tocommunicate bidirectionally among the one or more LPs 102, 104 and/orthe ICD 106 and transmit data including designated codes for eventsdetected or created by an individual LP. Individual LPs can beconfigured to issue a unique code corresponding to an event type and alocation of the sending pacemaker. While the LP 102, 104 shown in FIG.1B is shown as including only two electrodes 108, in alternativeembodiments discussed below, an LP can include more than two electrodes.

In some embodiments, an individual LP 102, 104 can be configured todeliver a pacing pulse with an event message encoded therein, with acode assigned according to pacemaker location and configured to transmita message to one or more other LPs via the event message coded pacingpulse. The pacemaker or pacemakers receiving the message are adapted torespond to the message in a predetermined manner depending on type andlocation of the event.

Moreover, information communicated on the incoming channel can alsoinclude an event message from another leadless cardiac pacemakersignifying that the other leadless cardiac pacemaker has sensed aheartbeat or has delivered a pacing pulse, and identifies the locationof the other pacemaker. For example, LP 104 may receive and relay anevent message from LP 102 to the programmer. Similarly, informationcommunicated on the outgoing channel can also include a message toanother LP, or to the ICD, that the sending leadless cardiac pacemakerhas sensed a heartbeat or has delivered a pacing pulse at the locationof the sending pacemaker.

Referring again to FIG. 1A, the cardiac pacing system 100 may comprisean implantable cardioverter-defibrillator (ICD) 106 in addition to LPs102, 104 configured for implantation in electrical contact with acardiac chamber and for performing cardiac rhythm management functionsin combination with the implantable ICD 106. The implantable ICD 106 andthe one or more LPs 102, 104 can be configured for leadlessintercommunication by information conduction through body tissue and/orwireless transmission between transmitters and receivers in accordancewith the discussed herein.

As shown in the illustrative embodiments, an LP 102, 104 can comprisetwo or more leadless electrodes 108 configured for delivering cardiacpacing pulses, sensing evoked and/or natural cardiac electrical signals,and bidirectionally communicating with the co-implanted ICD 106.

LP 102, 104 can be configured for operation in a particular location anda particular functionality at manufacture and/or at programming by anexternal programmer 109. Bidirectional communication among the multipleleadless cardiac pacemakers can be arranged to communicate notificationof a sensed heartbeat or delivered pacing pulse event and encoding typeand location of the event to another implanted pacemaker or pacemakers.LP 102, 104 receiving the communication decode the information andrespond depending on location of the receiving pacemaker andpredetermined system functionality.

In some embodiments, the LPs 102 and 104 are configured to beimplantable in any chamber of the heart, namely either atrium (RA, LA)or either ventricle (RV, LV). Furthermore, for dual-chamberconfigurations, multiple LPs may be co-implanted (e.g., one in the RAand one in the RV, or one in the RV and one in the coronary sinusproximate the LV). Certain pacemaker parameters and functions depend on(or assume) knowledge of the chamber in which the pacemaker is implanted(and thus with which the LP is interacting; e.g., pacing and/orsensing). Some non-limiting examples include: sensing sensitivity, anevoked response algorithm, use of AF suppression in a local chamber,blanking and refractory periods, etc. Accordingly, each LP preferablyknows an identity of the chamber in which the LP is implanted, andprocesses may be implemented to automatically identify a local chamberassociated with each LP.

Processes for chamber identification may also be applied to subcutaneouspacemakers, ICDs, with leads and the like. A device with one or moreimplanted leads, identification and/or confirmation of the chamber intowhich the lead was implanted could be useful in several pertinentscenarios. For example, for a DR or CRT device, automatic identificationand confirmation could mitigate against the possibility of the clinicianinadvertently placing the V lead into the A port of the implantablemedical device, and vice-versa. As another example, for an SR device,automatic identification of implanted chamber could enable the deviceand/or programmer to select and present the proper subset of pacingmodes (e.g., AAI or WI), and for the IPG to utilize the proper set ofsettings and algorithms (e.g., V-AutoCapture vs. ACap-Confirm, sensingsensitivities, etc.).

Also shown in FIG. 1B, the primary battery 114 has positive pole 140 andnegative pole 142. Current from the positive pole 140 of primary battery114 flows through a shunt 144 to a regulator circuit 146 to create apositive voltage supply 148 suitable for powering the remainingcircuitry of the pacemaker 102. The shunt 144 enables the batterycurrent monitor 136 to provide the controller 112 with an indication ofbattery current drain and indirectly of device health. The illustrativepower supply can be a primary battery 114.

In various embodiments, LP 102, 104 can manage power consumption to drawlimited power from the battery, thereby reducing device volume. Eachcircuit in the system can be designed to avoid large peak currents. Forexample, cardiac pacing can be achieved by discharging a tank capacitor(not shown) across the pacing electrodes. Recharging of the tankcapacitor is typically controlled by a charge pump circuit. In aparticular embodiment, the charge pump circuit is throttled to rechargethe tank capacitor at constant power from the battery.

In some embodiments, the controller 112 in one LP 102, 104 can accesssignals on the electrodes 108 and can examine output pulse duration fromanother pacemaker for usage as a signature for determining triggeringinformation validity and, for a signature arriving within predeterminedlimits, activating delivery of a pacing pulse following a predetermineddelay of zero or more milliseconds. The predetermined delay can bepreset at manufacture, programmed via an external programmer, ordetermined by adaptive monitoring to facilitate recognition of thetriggering signal and discriminating the triggering signal from noise.In some embodiments or in some conditions, the controller 112 canexamine output pulse waveform from another leadless cardiac pacemakerfor usage as a signature for determining triggering information validityand, for a signature arriving within predetermined limits, activatingdelivery of a pacing pulse following a predetermined delay of zero ormore milliseconds.

In certain embodiments, the electrodes of an LP 102, 104 can be used tosense an intracardiac electrocardiogram (IEGM) from which atrial and/orventricular activity can be detected, e.g., by detecting R waves and/orP waves. Accordingly, the sensed IEGM can be used by an LP to time itsdelivery of pacing pulses. Where an IEGM sensed by an LP is indicativeof electrical cardiac activity associated with the same cardiac chamberwithin or on which an LP is implanted, the IEGM can be referred to as anear-field signal. Where an IEGM sensed by an LP is indicative ofelectrical cardiac activity associate with another cardiac chamber ofthe heart (other than the cardiac chamber within or on which the LP isimplanted), the IEGM can be referred to as a far-field signal. An IEGMcan also be used by an LP 102, 104 to time when i2i communication pulsesshould be generated and transmitted, since the orientation of the LPs102, 104 relative to one another can change throughout each cardiaccycle.

FIG. 2 shows an LP 102, 104. The LP can include a hermetic housing 202(e.g., the housing 110 in FIG. 1) with electrodes 108 a and 108 bdisposed thereon. As shown, electrode 108 a can be separated from butsurrounded partially by a fixation mechanism 205, and the electrode 108b can be disposed on the housing 202. The fixation mechanism 205 can bea fixation helix, a plurality of hooks, barbs, or other attachingfeatures configured to attach the pacemaker to tissue, such as hearttissue. The electrodes 108 a and 108 b are examples of the electrodes108 shown in and discussed above with reference to FIG. 1B. One of theelectrodes 108 (e.g., 108 a) can function as a cathode type electrodeand another one of the electrodes 108 (e.g., 108 b) can function as ananode type electrode, or vice versa, when the electrodes are used fordelivering stimulation. The electrode 108 a is an example of a tipelectrode, and the electrode 108 b is an example or a ring electrode.The electrodes 108 a and 108 b can be referred to collectively as theelectrodes 108, or individually as the electrode 108. While the LP 102,104 shown in FIG. 2 is shown as including only two electrodes 108, inalternative embodiments discussed below, an LP can include more than twoelectrodes. The LP 102, 104 shown in FIG. 2 is also shown as including aretrieval feature 207, which can include a “button” or circular graspingfeature that is configured to dock within a docking cap or a retrievalcatheter that can be used to remove the LP 102, 104 when it needs to beremoved and/or replaced. Alternative form factors for the retrievalfeature are also possible.

Where an LP includes more than two electrodes, a first subset of theelectrodes can be used for delivering pacing pulses, a second subset ofthe electrodes can be used for sensing a near-field signal, a thirdsubset of the electrodes can be used for sensing a far-field signal, anda fourth subset of the electrodes can be used for transmitting andreceiving i2i messages. One or more of the first, second, third, andforth subsets of electrodes can be the same, or they can all differ fromone another. As used herein, the term near-field signal refers to asignal that originates in a local chamber (i.e., the same chamber)within which or on which corresponding sense electrodes (and the LPincluding the sense electrodes) are located. Conversely, the termfar-field signal refers to a signal that originates in a chamber otherthan the local chamber within which or on which corresponding senseelectrodes (and the LP including the sense electrodes) are located.

The housing 202 can also include an electronics compartment 210 withinthe housing that contains the electronic components necessary foroperation of the pacemaker, including, e.g., a pulse generator,receiver, and a processor for operation. The hermetic housing 202 can beadapted to be implanted on or in a human heart, and can be cylindricallyshaped, rectangular, spherical, or any other appropriate shapes, forexample.

The housing 202 can comprise a conductive, biocompatible, inert, andanodically safe material such as titanium, 316L stainless steel, orother similar materials. The housing 202 can further comprise aninsulator disposed on the conductive material to separate electrodes 108a and 108 b. The insulator can be an insulative coating on a portion ofthe housing between the electrodes, and can comprise materials such assilicone, polyurethane, parylene, or another biocompatible electricalinsulator commonly used for implantable medical devices. In theembodiment of FIG. 2, a single insulator 208 is disposed along theportion of the housing between electrodes 108 a and 108 b. In someembodiments, the housing itself can comprise an insulator instead of aconductor, such as an alumina ceramic or other similar materials, andthe electrodes can be disposed upon the housing.

As shown in FIG. 2, the pacemaker can further include a header assembly212 to isolate electrodes 108 a and 108 b. The header assembly 212 canbe made from PEEK, tecothane or another biocompatible plastic, and cancontain a ceramic to metal feedthrough, a glass to metal feedthrough, orother appropriate feedthrough insulator as known in the art.

The electrodes 108 a and 108 b can comprise pace/sense electrodes, orreturn electrodes. A low-polarization coating can be applied to theelectrodes, such as sintered platinum, platinum-iridium, iridium,iridium-oxide, titanium-nitride, carbon, or other materials commonlyused to reduce polarization effects, for example. In FIG. 2, electrode108 a can be a pace/sense electrode and electrode 108 b can be a returnelectrode. The electrode 108 b can be a portion of the conductivehousing 202 that does not include an insulator 208. As noted above, anddescribed in additional detail below, an LP can include more than twoelectrodes, and may use different combinations of the electrodes forsensing a near-field signal, sensing a far-field signal, deliveringpacing pulses, and sending and receiving i2i messages. When theelectrode 108 a is used as a pace electrode it can also be referred toas the cathode.

Several techniques and structures can be used for attaching the housing202 to the interior or exterior wall of the heart. A helical fixationmechanism 205, can enable insertion of the device endocardially orepicardially through a guiding catheter. A torqueable catheter can beused to rotate the housing and force the fixation device into hearttissue, thus affixing the fixation device (and also the electrode 108 ain FIG. 2) into contact with stimulable tissue. Electrode 108 b canserve as an indifferent electrode (also referred to as the anode) forsensing and pacing. The fixation mechanism may be coated partially or infull for electrical insulation, and a steroid-eluting matrix may beincluded on or near the device to minimize fibrotic reaction, as isknown in conventional pacing electrode-leads.

Implant-to-Implant (i2i) Event Messaging

LPs 102 and 104 can utilize implant-to-implant (i2i) communicationthrough event messages to coordinate operation with one another invarious manners. The terms i2i communication, i2i event messages, andi2i even markers are used interchangeably herein to refer to eventrelated messages and IMD/IMD operation related messages transmitted froman implanted device and directed to another implanted device (althoughexternal devices, e.g., a programmer, may also receive i2i eventmessages). In certain embodiments, LP 102 and LP 104 operate as twoindependent leadless pacers maintaining beat-to-beat dual-chamberfunctionality via a “Master/Slave” operational configuration. Fordescriptive purposes, the ventricular LP 104 shall be referred to as“vLP” and the atrial LP 102 shall be referred to as “aLP”. LP 102, 104that is designated as the master device (e.g. vLP) may implement all ormost dual-chamber diagnostic and therapy determination algorithms. Forpurposes of the following illustration, it is assumed that the vLP is a“master” device, while the aLP is a “slave” device. Alternatively, theaLP may be designated as the master device, while the vLP may bedesignated as the slave device. The master device orchestrates most orall decision-making and timing determinations (including, for example,rate-response changes).

In accordance with certain embodiments, methods are provided forcoordinating operation between first and second leadless pacemakers(LPs) configured to be implanted entirely within (or alternatively on)first and second chambers of the heart. A method transmits an eventmarker through conductive communication through electrodes located alonga housing of the first LP, the event marker indicative of one of a localpaced or sensed event. The method detects, over a sensing channel, theevent marker at the second LP. The method identifies the event marker atthe second LP based on a predetermined pattern configured to indicatethat an event of interest has occurred in a remote chamber. In responseto the identifying operation, the method initiates a related action inthe second LP.

FIG. 3 is a timing diagram 300 demonstrating one example of an i2icommunication for a paced event. The i2i communication may betransmitted, for example, from LP 102 to LP 104. As shown in FIG. 3, inthis embodiment, an i2i transmission 302 is sent prior to delivery of apace pulse 304 by the transmitting LP (e.g., LP 102). This enables thereceiving LP (e.g., LP 104) to prepare for the remote delivery of thepace pulse. The i2i transmission 302 includes an envelope 306 that mayinclude one or more individual pulses. For example, in this embodiment,envelope 306 includes a low frequency pulse 308 followed by a highfrequency pulse train 310. Low frequency pulse 308 lasts for a periodT_(i2iLF), and high frequency pulse train 310 lasts for a periodT_(i2iHF). The end of low frequency pulse 308 and the beginning of highfrequency pulse train 310 are separated by a gap period, T_(i2iGap).

As shown in FIG. 3, the i2i transmission 302 lasts for a period Ti2iP,and pace pulse 304 lasts for a period Tpace. The end of i2i transmission302 and the beginning of pace pulse 304 are separated by a delay period,TdelayP. The delay period may be, for example, between approximately 0.0and 10.0 milliseconds (ms), particularly between approximately 0.1 msand 2.0 ms, and more particularly approximately 1.0 ms. The termapproximately, as used herein, means+/−10% of a specified value.

FIG. 4 is a timing diagram 400 demonstrating one example of an i2icommunication for a sensed event. The i2i communication may betransmitted, for example, from LP 102 to LP 104. As shown in FIG. 4, inthis embodiment, the transmitting LP (e.g., LP 102) detects the sensedevent when a sensed intrinsic activation 402 crosses a sense threshold404. A predetermined delay period, T_(delayS), after the detection, thetransmitting LP transmits an i2i transmission 406 that lasts apredetermined period T_(i2iS). The delay period may be, for example,between approximately 0.0 and 10.0 milliseconds (ms), particularlybetween approximately 0.1 ms and 2.0 ms, and more particularlyapproximately 1.0 ms.

As with i2i transmission 302, i2i transmission 406 may include anenvelope that may include one or more individual pulses. For example,similar to envelope 406, the envelope of i2i transmission 406 mayinclude a low frequency pulse followed by a high frequency pulse train.

Optionally, wherein the first LP is located in an atrium and the secondLP is located in a ventricle, the first LP produces an AS/AP eventmarker to indicate that an atrial sensed (AS) event or atrial paced (AP)event has occurred or will occur in the immediate future. For example,the AS and AP event markers may be transmitted following thecorresponding AS or AP event. Alternatively, the first LP may transmitthe AP event marker slightly prior to delivering an atrial pacing pulse.Alternatively, wherein the first LP is located in an atrium and thesecond LP is located in a ventricle, the second LP initiates anatrioventricular (AV) interval after receiving an AS or AP event markerfrom the first LP; and initiates a post atrial ventricular blanking(PAVB) interval after receiving an AP event marker from the first LP.

Optionally, the first and second LPs may operate in a “pure”master/slave relation, where the master LP delivers “command” markers inaddition to or in place of “event” markers. A command marker directs theslave LP to perform an action such as to deliver a pacing pulse and thelike. For example, when a slave LP is located in an atrium and a masterLP is located in a ventricle, in a pure master/slave relation, the slaveLP delivers an immediate pacing pulse to the atrium when receiving an APcommand marker from the master LP.

In accordance with some embodiments, communication and synchronizationbetween the aLP and vLP is implemented via conducted communication ofmarkers/commands in the event messages (per i2i communication protocol).As explained above, conducted communication represents event messagestransmitted from the sensing/pacing electrodes at frequencies outsidethe RF or Wi-Fi frequency range. Alternatively, the event messages maybe conveyed over communication channels operating in the RF or Wi-Fifrequency range. The figures and corresponding description belowillustrate non-limiting examples of markers that may be transmitted inevent messages. The figures and corresponding description below alsoinclude the description of the markers and examples of results thatoccur in the LP that receives the event message. Table 2 representsexemplary event markers sent from the aLP to the vLP, while Table 3represents exemplary event markers sent from the vLP to the aLP. In themaster/slave configuration, AS event markers are sent from the aLP eachtime that an atrial event is sensed outside of the post ventricularatrial blanking (PVAB) interval or some other alternatively-definedatrial blanking period. The AP event markers are sent from the aLP eachtime that the aLP delivers a pacing pulse in the atrium. The aLP mayrestrict transmission of AS markers, whereby the aLP transmits AS eventmarkers when atrial events are sensed both outside of the PVAB intervaland outside the post ventricular atrial refractory period (PVARP) orsome other alternatively-defined atrial refractory period.Alternatively, the aLP may not restrict transmission of AS event markersbased on the PVARP, but instead transmit the AS event marker every timean atrial event is sensed.

TABLE 2 “A2V” Markers/Commands (i.e., from aLP to vLP) MarkerDescription Result in vLP AS Notification of a sensed event Initiate AVinterval (if not in in atrium (if not in PVAB or PVAB or PVARP) PVARP)AP Notification of a paced event in Initiate PAVB atrium Initiate AVinterval (if not in PVARP)

As shown in Table 2, when an aLP transmits an event message thatincludes an AS event marker (indicating that the aLP sensed an intrinsicatrial event), the vLP initiates an AV interval timer. If the aLPtransmits an AS event marker for all sensed events, then the vLP wouldpreferably first determine that a PVAB or PVARP interval is not activebefore initiating an AV interval timer. If however the aLP transmits anAS event marker only when an intrinsic signal is sensed outside of aPVAB or PVARP interval, then the vLP could initiate the AV intervaltimer upon receiving an AS event marker without first checking the PVABor PVARP status. When the aLP transmits an AP event marker (indicatingthat the aLP delivered or is about to deliver a pace pulse to theatrium), the vLP initiates a PVAB timer and an AV interval time,provided that a PVARP interval is not active. The vLP may also blank itssense amplifiers to prevent possible crosstalk sensing of the remotepace pulse delivered by the aLP.

TABLE 3 “V2A” Markers/Commands (i.e., from vLP to aLP) MarkerDescription Result in aLP VS Notification of a sensed event InitiatePVARP in ventricle VP Notification of a paced event in Initiate PVABventricle Initiate PVARP AP Command to deliver Deliver immediate paceimmediate pace pulse in pulse to atrium atrium

As shown in Table 3, when the vLP senses a ventricular event, the vLPtransmits an event message including a VS event marker, in response towhich the aLP may initiate a PVARP interval timer. When the vLP deliversor is about to deliver a pace pulse in the ventricle, the vLP transmitsVP event marker. When the aLP receives the VP event marker, the aLPinitiates the PVAB interval timer and also the PVARP interval timer. TheaLP may also blank its sense amplifiers to prevent possible crosstalksensing of the remote pace pulse delivered by the vLP. The vLP may alsotransmit an event message containing an AP command marker to command theaLP to deliver an immediate pacing pulse in the atrium upon receipt ofthe command without delay.

The foregoing event markers are examples of a subset of markers that maybe used to enable the aLP and vLP to maintain full dual chamberfunctionality. In one embodiment, the vLP may perform all dual-chamberalgorithms, while the aLP may perform atrial-based hardware-relatedfunctions, such as PVAB, implemented locally within the aLP. In thisembodiment, the aLP is effectively treated as a remote ‘wireless’ atrialpace/sense electrode. In another embodiment, the vLP may perform mostbut not all dual-chamber algorithms, while the aLP may perform a subsetof diagnostic and therapeutic algorithms. In an alternative embodiment,vLP and aLP may equally perform diagnostic and therapeutic algorithms.In certain embodiments, decision responsibilities may be partitionedseparately to one of the aLP or vLP. In other embodiments, decisionresponsibilities may involve joint inputs and responsibilities.

In the event that LP to LP (i2i) communication is lost (prolonged ortransient), the system 100 may automatically revert to safeventricular-based pace/sense functionalities as the vLP device isrunning all of the necessary algorithms to independently achieve thesefunctionalities. For example, if the vLP loses i2i communication it mayrevert from the VDD mode to a VVI mode or a VDI mode, and if the aLPloses i2i communication it may revert from ADD mode to an OAO mode or anAAI mode. Thereafter, once i2i communication is restored, the system 100can automatically resume dual-chamber functionalities.

As noted above, when using a pair of LPs (e.g., 102, 104) to performpacing and/or sensing operations in the RA and RV, one of the challengesis that i2i communication may be relied upon to maintain appropriatesynchrony between the RV and the RA.

As also noted above, a transmitter (e.g., 118) of an LP 102, 104 may beconfigured to transmit event messages in a manner that does notinadvertently capture the heart in the chamber where LP 102, 104 islocated, such as when the associated chamber is not in a refractorystate. In addition, an LP 102, 104 that receives an event message mayenter an “event refractory” state (or event blanking state) followingreceipt of the event message. The event refractory/blanking state may beset to extend for a determined period of time after receipt of an eventmessage in order to avoid the receiving LP 102, 104 from inadvertentlysensing another signal as an event message that might otherwise causeretriggering. For example, the receiving LP 102, 104 may detect ameasurement pulse from another LP 102, 104. The amplitude of a detected(i.e., sensed) measurement pulse can be referred to as the sensedamplitude.

As noted above, it has been observed that i2i communication can beadversely affected by the orientation of the LPs relative to oneanother. Both computer simulations and animal testing have showed thatsensed i2i amplitude varied widely with different orientation angles.For example, where a first LP (e.g., 102) transmits a pulse having apulse amplitude of 2.5V to a second LP (e.g., 104), the sensed amplitudeof the pulse received by the second LP (e.g., 104) could vary from about2 mV to less than 0.5 mV, depending upon the orientation between thefirst and second LPs (e.g., 102 and 104). For example, where the LP 102is implanted in or on the RA chamber, and the LP 104 is implanted in oron the RV chamber, e.g., as shown in FIG. 1A, the orientation of the LPs102 and 104 relative to one another can change over the course of eachcardiac cycle. Additionally, the orientation of the LPs 102 and 104relative to one another can be affected by the posture of the patient.Accordingly, since the sensed amplitude of an i2i pulse received by oneLP (e.g., 104) from the other LP (e.g., 102) can significantly varybased on the orientation of the LPs relative to one another, the sensei2i amplitude can significantly vary depending upon the timing of whenan i2i pulse is transmitted during a cardiac cycle, as well as theposture of the patient when the pulse is transmitted.

Assume, for example, that an LP 102, 104 has a 0.5 mV i2i sensethreshold, meaning that a sensed pulse must have an amplitude of atleast 0.5 mV in order to be detected as a communication pulse by thereceiving LP. In other words, if sensed amplitudes of receivedcommunication pulses are below the sense threshold, the receiving LPwill fail to receive the information encoded therein and may fail torespond accordingly, which is undesirable.

FIG. 5 is a diagram that is used to show how the orientation of twodifferent LPs (e.g., 102, 104), labeled LP2 and LP1 in FIG. 5, can bequantified. Referring to FIG. 5, the LP2 (e.g., 102) is shown as havingan axis 502, and the LP1 (e.g., 104) is shown as having an axis 504. Theline D12 represents the distance between the LP1 and the LP2. In FIG. 5,the angle α12 is the angle between the axis 504 of the LP1 and the lineD12; the angle β12 is the angle between the axis 502 of the LP2 and theline D12; and the angle γ12 is angle between the plane defined by theangle α12 and the plane defined by the angle 1312.

Table 3, below, provides the results of simulations that show how sensedamplitudes are affected by the orientation of LP1 and LP2 relative toone another, where the LP2 is assumed to be implanted in the RA chamber,the LP1 is assumed to be implanted in the RV chamber, and the distanceD12 is assumed to be fixed at 124 millimeters (mm).

TABLE 4 Dis- tance Angle Angle D12 α12 β12 RA → RV RA ← RV 124 mm 20°12° 2.5 V → 2.13 mV 2.11 mV ←2.5 V 124 mm 20° 32° 2.5 V → 1.82 mV N/A124 mm 20° 52° 2.5 V → 1.32 mV N/A 124 mm 20° 72° 2.5 V → 0.745 mV N/A124 mm 20° 82° 2.5 V → 0.470 mV 0.460 mV ←2.5 V 124 mm 20° 92° 2.5 V →0.198 mV 0.198 mV ←2.5 V 124 mm 10° 82° 2.5 V → 0.6627 mV N/A 124 mm 40°82° 2.5 V → −0.1135 mV N/A

The first row of Table 4 shows that when the angle 1312 (i.e., the anglebetween the axis 502 of the LP2 and the line D12) is 12 degrees, inresponse to the LP2 transmitting a communication pulse having anamplitude of 2.5V, the sense amplitude of the communication pulsereceived by the LP1 will be 2.13 mV, which is well above a 0.5 mV sensethreshold. By contrast, the sixth row of Table 4 shows that when theangle β12 is 92 degrees, in response to the LP2 transmitting acommunication pulse having an amplitude of 2.5V, the sense amplitude ofthe communication pulse received by the LP1 will be only 0.198 mV, whichis well below the 0.5 mV sense threshold. Looking at the right mostcolumn and the first row of Table 4 shows that when the angle β12 is 12degrees, in response to the LP1 transmitting a communication pulsehaving an amplitude of 2.5V, the sense amplitude of the communicationpulse received by the LP2 will be 2.11 mV, which is well above a 0.5 mVsense threshold; and when the angle β12 is 92 degrees, in response tothe LP1 transmitting a communication pulse having an amplitude of 2.5V,the sense amplitude of the communication pulse received by the LP2 willbe only 0.198 mV, which is well below the 0.5 mV sense threshold.

With larger heart sizes, the sensed amplitudes decrease. Morespecifically, a larger heart can cause the distance D12 between the LP1and the LP2 to increase, with the results summarized in Table 5, below.

TABLE 5 Dis- tance Angle Angle D12 α12 β12 RA → RV RA ← RV 150 mm 20°12° 2.5 V → 0.96 mV N/A 150 mm 20° 32° 2.5 V → 0.76 mV N/A 150 mm 20°52° 2.5 V → 0.51 mV N/A 150 mm 20° 72° 2.5 V → 0.25 mV N/A 150 mm 20°82° 2.5 V → 0.12 mV N/A 150 mm 20° 92° 2.5 V → 0.005 mV N/A 150 mm 20°52° 2.5 V → 0.51 mV N/A 150 mm 10° 52° 2.5 V → 0.59 mV N/A 150 mm 40°52° 2.5 V → 0.27 mV N/A

The results summarized in Table 5 mimic a worst case where the heartsize is at the upper bounds (D12˜150 mm). As can be appreciated from acomparison between Table 5 and Table 4, the sensed amplitudes decreasedas D12 was increased from 124 mm to 150 mm, so that in Table 5 when theangle β12 is greater than 52 degree, the sensed amplitude is lower thanthe 0.5 mV sense threshold. Accordingly, it can be appreciated that i2icommunications between LPs implanted in larger hearts are even moreadversely affected than smaller hearts by the relative orientation ofthe LPs.

When performing i2i communication, the one or more pulses that aretransmitted from one LP to another LP can be referred more generally asthe i2i signal. Due to the nature of electrode potential distribution,bipolar sensing of the i2i signal (by the LP that is receiving/sensingthe i2i signal) is minimal along iso-potential lines and maximum alonglines orthogonal to the iso-potential lines. In other words, when therespective axes (e.g., 502 and 504 in FIG. 5) of the two LPs(communicating with one another) are aligned with one another the sensedi2i signal is near its maximum, and when the respective axes (e.g., 502and 504 in FIG. 5) of the two LPs are orthogonal to one another thesensed i2i signal is near its minimum.

For the purpose of this discussion, when the LPs are oriented relativeto another such that (for a give transmitted communication pulseamplitude) the sense amplitude of the communication pulse received by anLP will be below the sense threshold (e.g., 0.5 mV), the LPs can be saidto be within a “deaf zone”. This is because under such circumstances theLPs cannot successfully communicate or “hear” one another even thoughthey are attempting to communicate or “talk” with one another.

Use of Far-Field and/or Sensor Signals to Supplement or Replace i2iMessaging

In accordance with certain embodiments of the present technology, one ormore accelerometers of an LP can be used to determine when the LP islikely in a deaf zone, and during such periods the LP can rely onfar-field sensing to time delivery of pacing within the chamber in whichthe LP is implanted. For example, the aLP can sense a far-field signalfrom which electrical cardiac activity associated with the RV chambercan be detected, and the aLP can perform ADD pacing by timing deliveryof atrial pulses based on the timing of cardiac activity associated withthe RV chamber as detected from the far-field signal. For anotherexample, the vLP can sense a far-field signal from which electricalcardiac activity associated with the RA chamber can be detected, and thevLP can perform VDD pacing by timing delivery of ventricular pulsesbased on the timing of cardiac activity associated with the RA chamberas detected from the far-field signal. When an aLP implanted in the RAchamber times delivery of atrial pacing pulses based on the timing of RVcardiac activity as detected from a far-field signal sensed by the aLP,it can be said that the aLP times its delivery of atrial pacing pulsesbased on timing of RV cardiac activity detected by the aLP itself.Similarly, when an vLP implanted in the RV chamber times delivery ofventricular pacing pulses based on the timing of RA cardiac activity asdetected from a far-field signal sensed by the vLP, it can be said thatthe vLP times its delivery of ventricular pacing pulses based on timingof RA cardiac activity as detected by the vLP itself.

Depending upon the specific implementation, an aLP can primarily or bydefault time delivery of RA pacing pulses based on the timing of cardiacactivity associated with the RV chamber as determined based on i2imessages received by the aLP from a vLP, and the aLP can, as a backup,time delivery of RA pacing pulses based on the timing of cardiacactivity associated with the RV chamber that the aLP detected itselffrom a far-field signal that the aLP sensed. Similarly, a vLP canprimarily or by default time delivery of RV pacing pulses based on thetiming of cardiac activity associated with the RA chamber as determinedbased on i2i messages received by the vLP from an aLP, and the vLP can,as a backup, time delivery of RV pacing pulses based on the timing ofcardiac activity associated with the RA chamber that the vLP detecteditself from a far-field signal that the vLP sensed.

Alternatively, an aLP can primarily or by default time delivery of RApacing pulses based on the timing of cardiac activity associated withthe RV chamber as determined based on a far-field signal that the aLPsensed itself, and the aLP can, as a backup, time delivery of RA pacingpulses based on the timing of cardiac activity associated with the RVchamber as determined based on i2i messages received by the aLP from avLP. Similarly, a vLP can primarily or by default time delivery of RVpacing pulses based on the timing of cardiac activity associated withthe RA chamber as determined based on a far-field signal that the vLPsensed itself, and the vLP can, as a backup, time delivery of RV pacingpulses based on the timing of cardiac activity associated with the RAchamber as determined based on i2i messages received by the vLP from anaLP.

In accordance with certain embodiments of the present technology,instead of (or in addition to) an LP detecting electrical cardiacactivity associated with another chamber based on a far-field signalsensed by the LP itself, the LP can use a sensor (e.g., an accelerometeror pressure sensor) to produce a sensor signal from which mechanicalcardiac activity associated with another chamber of the heart may bedetected. For example, the aLP can use an accelerometer or pressuresensor to produce a sensor signal from which heart sounds associatedwith the RV chamber can be detected, and the aLP can perform ADD pacingby timing delivery of atrial pulses based on the timing of cardiacactivity associated with the RV chamber as detected from the sensorsignal. For another example, the vLP can use an accelerometer orpressure sensor to produce a sensor signal from which mechanical cardiacactivity associated with the RA chamber can be detected, and the vLP canperform VDD pacing by timing delivery of ventricular pulses based on thetiming of cardiac activity associated with the RA chamber as detectedfrom the sensor signal.

Where the vLP performs VDD pacing, it performs ventricular pacing,atrial and ventricular (i.e., dual) sensing, dual (i.e., triggered andinhibited) response to a sensed event. VDD may be used, e.g., for AVnodal dysfunction but intact and appropriate sinus node behavior. Theventricular sensing can be based on a near-field signal that the vLPsenses itself using a pair of its electrodes. Similarly, if the vLPperforms WI or VDI pacing, it can perform ventricular sensing based on anear-field signal that the vLP senses itself using a pair of itselectrodes. Atrial sensing can be based on a far-field signal that thevLP senses itself using a pair of its electrodes, based on a sensorsignal that vLP senses itself (e.g., using an accelerometer or pressuresensor of the vLP), and/or based on i2i messages that the vLP receivesfrom the aLP.

Where the aLP performed ADD pacing, it performs atrial pacing, atrialand ventricular (i.e., dual) sensing, dual (i.e., triggered andinhibited) response to a sensed event. The atrial sensing can be basedon a near-field signal that the aLP senses itself using a pair of itselectrodes. Similarly, if the aLP performs AAI or ADI pacing, it canperform atrial sensing based on a near-field signal that the aLP sensesitself using a pair of its electrodes. Ventricular sensing can be basedon a far-field signal that the aLP senses itself using a pair of itselectrodes, based on a sensor signal that aLP senses itself (e.g., usingan accelerometer or pressure sensor of the aLP), and/or based on i2imessages that the aLP receives from the vLP.

The high level flow diagram of FIG. 6 will now be used to describecertain embodiments of the present technology for use with animplantable system including a first leadless pacemaker (LP1) implantedin or on a first chamber of a heart and a second leadless pacemaker(LP2) implanted in or on a second chamber of the heart, wherein the LP1includes a plurality of electrodes at least two of which can be used bythe LP1 to transmit and receive implant-to-implant (i2i) messages to andfrom the LP2. More specifically, the high level flow diagram of FIG. 6will be used to describe a method for use by the LP1 that is implantedin or on the first chamber of the heart. For the embodiments describedwith reference to FIG. 6, it is assumed that the LP1 preferably or bydefault attempts to time its pacing pulses (relative to activity of aremote chamber) based on a sensed far-field signal, and that i2icommunication is used as a backup when the electrical cardiac activityassociated with the second chamber of the heart is not successfullydetected based on the far-field signal.

Referring to FIG. 6, at step 602 the LP1 (implanted in or on the firstchamber of the heart) attempts to sense a far-field signal from whichelectrical cardiac activity associated with the second chamber of theheart may be detected. At step 604 there is a determination of whetherthe LP1 successfully detects electrical cardiac activity associated withthe second chamber of the heart based on the far-field signal. There arevarious different ways that the determination at step 604 can beperformed, depending on the specific implementation, as well as based onwhere the LP1 and LP2 are implanted. For example, if the first chamberof the heart (within or on which the LP1 is implanted) is the RVchamber, and the second chamber of the heart (for which cardiac activityis trying to be detected from far-field signal sensed by the LP1) is theRA chamber, then step 604 can be performed by determining whetherindicators of RA chamber contractions (e.g., A or P waves) aresuccessfully detected from the sensed far-field signal, and/or whetherthe SNR of the sensed far-field signal exceeds a specified threshold.For another example, if the first chamber of the heart (within which theLP1 is implanted) is the RA chamber, and the second chamber of the heart(for which cardiac activity is trying to be detected from far-fieldsignal sensed by the LP1) is the RV chamber, then step 604 can beperformed by determining whether indicators of RV chamber contractions(e.g., V or R waves) are successfully detected from the sensed far-fieldsignal, and/or whether the SNR of the sensed far-field signal exceeds aspecified threshold. These are just a few examples of how step 604 canbe performed, which are not intended to be all encompassing.

If the answer to the determination is Yes, then flow goes to step 606,as shown in FIG. 6. At step 606 the LP1 times delivery of one or morepacing pulses to the first chamber of the heart (within or on which theLP1 is implanted) based on timing of electrical cardiac activityassociated with the second chamber of the heart as detected from thefar-field signal that the LP1 sensed itself. For example, if the firstchamber of the heart (within or on which the LP1 is implanted) is the RVchamber, and the second chamber of the heart (for which cardiac activityis being detected from far-field signal sensed by the LP1) is the RAchamber, then step 606 can involve the LP1 timing delivery of one ormore pacing pulses to the RV chamber based on the timing of A or P wavesdetected from the far-field signal that the LP1 sensed (and based on aprogrammed AV delay). For another example, if the first chamber of theheart (within or on which the LP1 is implanted) is the RA chamber, andthe second chamber of the heart (for which cardiac activity is beingdetected from far-field signal sensed by the LP1) is the RV chamber,then step 606 can involve the LP1 timing delivery of one or more pacingpulses to the RA chamber based on the timing of V or R waves detectedfrom the far-field signal that the LP1 sensed (and based on a programmedVA interval). These are just a few examples of how step 606 can beperformed, which are not intended to be all encompassing.

If the answer to the determination is No, then flow goes to step 608, asshown in FIG. 6. At step 608 the LP1 times delivery of one or morepacing pulses delivered to the first chamber of the heart (within or onwhich the LP1 is implanted) based on timing of cardiac activityassociated with the second chamber of the heart as determined by the LP1based on i2i messages that the LP1 receives from the LP2 (implantedwithin or on the second chamber of the heart). The LP2 can send such i2imessages to the LP1 once per heartbeat, or the LP2 can send such i2imessages to the LP1 in response to a request from the LP1. Othervariations are also possible.

As can be appreciated from steps 702, 704, 706 and 708 shown in the highlevel flow diagram of FIG. 7, instead of (or in addition to) the LP1(implanted in or on the first chamber of the heart) attempting to sensea far-field signal from which electrical cardiac activity associatedwith the second chamber of the heart may be detected, the LP1 can use asensor of the LP1 to sense a sensor signal from which mechanical cardiacactivity associated with the second chamber of the heart may bedetected. In such an embodiment, the LP1 can use the sensor signal(instead of, or in addition to the far-field signal) to time delivery ofpacing to the first chamber of the heart (within or on which the LP1 isimplanted). For example, the sensor can be an accelerometer (e.g., 154)or a pressure sensor (e.g., 156 in FIG. 1B) from which heart soundsand/or other indicators of mechanical cardiac activity may be detected.Heart sounds are the noises generated by the beating heart and theresultant flow of blood, and are typically referred to as S1, S2, S3 andS4. Depending upon which heart sound is being detected, the LP1 canappropriately time its pacing therapy.

The S1 heart sound, which is typically the loudest and most detectableof the heart sounds, is caused by the sudden block of reverse blood flowdue to closure of the atrioventricular valves (mitral and tricuspid) atthe beginning of ventricular contraction. Isovolumic relaxation (IR)occurs during ventricular diastole and is demarcated approximately byclosure of the aortic valve and the second heart sound (S2) andapproximately by opening of the mitral valve and the third heart sound(S3), which is more prominent in children and those with abnormalventricular function when compared to normal adults. The onset ofisovolumic relaxation time commences with aortic valve closure, whichcan be identified by the aortic component (A2) of the second heart sound(S2). The third heart sound (S3) has been linked to flow between theleft atrium and the left ventricle, more generally LV filling, andthought to be due to cardiohemic vibrations powered by rapiddeceleration of transmitral blood flow. The fourth heart sound (S4) maybe present in the late stage of diastole and associated with atrialcontraction, or kick, where the final 20% of the atrial output isdelivered to the ventricles.

For the embodiments described with reference to FIG. 8, it is assumedthat the LP1 preferably or by default attempts to time its pacing pulses(relative to activity of a remote chamber) based on i2i communication,and uses a sensed far-field signal (from which electrical cardiacactivity may be detected) and/or a sensor signal (from which mechanicalcardiac activity may be detected) as a backup when valid i2i messagesare not detected from the LP2. Referring to FIG. 8, at step 802 the LP1(implanted in or on a first chamber of heart) attempts to receive fromthe LP2 (implanted in or on a second chamber of heart) i2i messages fromwhich cardiac activity associated with second chamber of heart may bedetected.

At step 804 there is a determination of whether the LP1 has successfullydetected valid i2i message(s) from the LP2. The term “i2i message”, asused herein, can refer to an actual i2i sent message that is receivedand is capable of being decoded, an actual sent i2i message that isreceived but is too noisy to be decoded, an actual sent i2i message thatis received but due to noise it is decoded mistakenly for a differenti2i message, noise that is initially mistaken for being an actual i2imessage but is sufficiently different than an actual i2i message so thatit cannot be decoded, as well as noise that is received and is mistakenfor being an actual i2i message and is decoded by the LP because thenoise is sufficiently similar to an actual i2i message. The term “validi2i message”, as used herein, refers to an actual sent i2i message thatis received and is capable of being decoded. In accordance with certainembodiments, the determination of whether an i2i message is valid orinvalid can be performed by a processor or controller that performserror detection or correction.

If the answer to the determination at step 804 is Yes, then flow goes tostep 806, as shown in FIG. 8. At step 806, the LP1 times delivery of oneor more pacing pulses to the first chamber of the heart (within or onwhich the LP1 is implanted) based on cardiac activity associated withthe second chamber of the heart determined from the received i2imessage(s). For example, if the first chamber of the heart (within or onwhich the LP1 is implanted) is the RV chamber, and the second chamber ofthe heart (within or on which the LP2 is implanted) is the RA chamber,then step 806 can involve the LP1 timing delivery of one or more pacingpulses to the RV chamber based on the timing of atrial activity asdetermined from i2i messages received from the LP2 (and based on aprogrammed AV delay). For another example, if the first chamber of theheart (within or on which the LP1 is implanted) is the RA chamber, andthe second chamber of the heart (within or on which the LP2 isimplanted) is the RV chamber, then step 806 can involve the LP1 timingdelivery of one or more pacing pulses to the RA chamber based on thetiming of ventricular activity as determined from i2i messages receivedfrom the LP2 (and based on a programmed VA interval). These are just afew examples of how step 606 can be performed, which are not intended tobe all encompassing. If the answer to the determination at step 804 isNo, then flow goes to step 808, as shown in FIG. 8. Step 808 it similarto step 602 and/or step 702 described above, and thus, need not bedescribed in detail again.

In still other embodiments, the LP1 (that is implanted within or on thefirst chamber of the heart) can test and compare use of a far-fieldsignal, a sensor signal, and i2i messages, for use in timing delivery ofone or more pacing pulses delivered to the first chamber of the heart bythe LP1. Such testing can be performed every heartbeat, periodically, orin response to a triggering event, such as in response to signal qualityof a selected signal falling below a specified level, or the like. Basedon the results of the testing and comparing, the LP1 can select to useeither the far-field signal, the sensor signal, or the i2i messages, foruse in timing delivery of one or more pacing pulses delivered to thefirst chamber of the heart by the LP1. For example, the relativestrength and/or robustness of a sensed far-field signal, i2i messages,and/or heart sounds can be used to select what type of signal localcardiac pacing should be based on. Strengths based on SNRs,discrimination capability, etc., can be used. It would also be possibleto just test and compare two of the above three options to select one ofthe two tested and compared options for timing delivering of pacingpulse(s) to the local chamber of the heart by the LP1.

It would also be possible for the LP1 to select between timingdelivering of its pacing pulses based on a far-field signal (and/or asensor signal) and timing delivery of pacing pulses based on i2imessages, wherein the selecting is based on one or more accelerometeroutputs indicative of the orientation of the LP1. For example, when theLP1 determines it's within or close to the deaf zone, it can choose totime delivery of pacing pulses based on a far-field signal (and/or asensor signal), rather than relying on i2i messages. Conversely, whenthe LP1 is not within or close to the deaf zone, it can choose to timedelivery of pacing pulses based on i2i message(s). Other variations arealso possible.

In many of the embodiments described above, an implantable cardiacpacing system was described as including an LP implanted within or onthe RA chamber and another LP implanted within or on the RV chamber. Itwould also be possible that one LP is implanted in or on the RV chamberand another LP is implanted in or on the LV chamber to allow forbi-ventricular pacing. It would also be possible for three LPs to beimplanted, such that one is in the RV chamber, one is in the LV chamber,and another is in the RA chamber, to allow for CRT. In still otherembodiments, it would also be possible to implant an LP in the LAchamber. Each of the LPs can itself detect cardiac activity associatewith one or more other chamber by sensing a far-field signal from whichelectrical cardiac activity associated with one or more other chambermay be detected, and/or by using a sensor to produce a sensor signalfrom which mechanical cardiac activity associated with one or more otherchamber may be detected. Each LP may also receive i2i messages from oneor more other LP(s) implanted in or on one or more other cardiacchambers and can time their pacing pulses based on information learnedfrom the i2i message. Accordingly, embodiments described herein, unlessstated otherwise, should not be limited to LPs implanted in the RAand/or RV chambers.

In many of the embodiments discussed above, each LP was described asincluding two electrodes. However, an LP can include more than twoelectrodes. Exemplary LPs that include more than two electrodes aredescribed below, e.g., with reference to FIGS. 9A and 9B. Were an LPincludes more than two electrodes, the electrodes that the LP uses tosense a far-field signal can be the same as, or different than, theelectrodes that the LP uses to transmit and receive i2i messages, aswill be described in additional detail below. Each such i2i messages canbe made up of one or more i2i pulses, exemplary details of which weredescribed above.

Leadless Pacemaker (LP) Implementations

Certain embodiments of the present technology, described below withreference to FIGS. 9A and 9B, are related to specific implementations ofan LP that enables the LP to effectively deliver pacing pulses to thecardiac chamber within or on which the LP is implanted, effectivelysense near-field signals, as well as effectively sense far-fieldsignals. A near-field signal can be used by the LP that senses thenear-field signal to monitor electrical cardiac activity of the cardiacchamber within or on which the LP is implanted, which cardiac chambercan be referred to as the local chamber. A far-field signal can be usedby the LP that senses the far-field signal to monitor electrical cardiacactivity associated with another chamber of the heart (within or onwhich the LP that obtains the far-field signal is not implanted). Suchother cardiac chamber of the heart (within or on which an LP is notimplanted) can also be referred to herein as a remote chamber. The LPcan also perform i2i communications.

Referring to FIG. 9A, an LP 902 a, according to an embodiment of thepresent technology, is shown therein. The LP 902 a is shown as includingtwo separate housings 912 and 922, each of which is made of anelectrically conductive material. The housings 912 and 922 can be madeof the same type of electrically conductive material as one another, orof different types of electrically conductive materials than oneanother. The electrically conductive material of which the housing 912and/or the housing 922 is made can be an electrically conductivebiocompatible metal or alloy, such as stainless steel, a cobalt-chromiumalloy, titanium, or a titanium alloy, but is not limited thereto. Itwould also be possible for the electrically conductive material of whichthe housing 912 and/or the housing 922 is made to be a currentlydeveloped or future developed electrically conductive polymericmaterial. Since the housings 912 and 922 are each made of anelectrically conductive material, they can also be referred to aselectrically conductive housings 912 and 922.

As shown in FIG. 9A, electronic circuitry 914 (also referred to aselectronics) is included within the housing 912, and a battery 924(e.g., the battery 114 in FIG. 1B) is located within the housing 922.The electronic circuitry 912 can include, e.g., one or more pulsegenerators, one or more sense amplifiers, switches, a controller,memory, and/or the like. Such a controller can include one or moreprocessors, and/or an application-specific integrated circuit (ASIC),but is not limited thereto. Exemplary details of the electroniccircuitry 912 were discussed above with reference to FIG. 1B, and arealso discussed below with reference to FIG. 11. Since the electroniccircuitry 912 can likely be made smaller in size that the battery 924(which should preferably power the LP for a few years), it is likelythat the housing 924 which encases the battery 924 is larger in volumethan the housing 912 which encases the electronic circuitry 914. Thehousings 912 and 922 are preferably hermetic housings that protect theelectronic circuitry 914 and the battery 924 from the harsh environmentof the human body.

Still referring to FIG. 9A, an inter-housing insulator 932 is locatedbetween the housing 912 and the housing 922 to electrically isolate thehousings 912 and 914 from one another. The inter-housing insulator 932can be made, e.g., of sapphire, ruby, a biocompatible glass (e.g.,borosilicate glass) or a biocompatible ceramic, but is not limitedthereto. Exemplary biocompatible ceramics include, but is not limitedaluminum nitride (AlN), zirconia (ZrO2), silicon carbide (SiC), andsilicon nitride (Si3N4).

In certain embodiments each of the housings 912 and 922 has a generallycylindrical shape. As shown in FIG. 9A, each of the housings 912 and 922has an end that is connected to the inter-housing insulator 932, and anopposing end that can be referred to as the “free end” of the respectivehousing. The end of each housing 912 and 922 that is not the free endcan be referred to as the “non-free end”. The non-free end of thehousing 912 can be physically attached to a first side of theinter-housing insulator 932. Similarly, the non-free end of the housing922 can be physically attached to a second side of the inter-housinginsulator 932. Where each of the housings 912 and 922 has a generallycylindrical shape the inter-housing insulator may have an annular shapeor a disk-like shape, but is not limited thereto. For more specificexamples, the inter-housing insulator 932 can be an annular shapedceramic or glass collar.

Preferably there is a hermetic bonding between the non-free end of thehousing 912 and the first side of the inter-housing insulator 932, and ahermetic bonding between the non-free end of the housing 922 and thesecond side of the inter-housing insulator 932. For example, fusionwelding methods, such as laser welding, tungsten inert gas welding(TIG), or electron-beam welding can be used to hermetically bond ends ofeach of the first and second housings 912 and 922 to the first andsecond sides of the inter-housing insulator 932. In certain embodiments,multiple hermetic seals are provided between an end of a housing (912 or922) and a side of the inter-housing insulator. Such hermetic seals caninclude, e.g., one or more glass-to-tantalum seals produced by meltingglass with an infrared laser beam, and one or more hermetic sealsobtained by melting a tantalum tube closed in a plasma needle arcwelder, but are not limited thereto.

The battery 924 can be any one of various different types of batteries,such as, but not limited to, a lithium battery, e.g., a lithium carbonmonofluoride (Li-CFx) battery. The use of other types of batteries isalso possible and within the scope of the embodiments described herein.The battery 924 has a positive (+) pole and a negative (−) pole. Inaccordance with certain embodiments where the battery 924 is an Li-CFxbattery, lithium (Li) provides the anode or negative (−) pole of thebattery, and carbon monofluoride (CFx) provides the cathode or positive(+) pole of the battery. When referring to the battery 924, the positive(+) pole can also be referred to as the positive (+) terminal, and thenegative (−) pole can also be referred to as the negative (−) terminal.

In accordance with certain embodiments, the negative (−) pole of thebattery 924 is connected to the electrically conductive housing 922 thatencases the battery 924. The connection between the negative (−) pole ofthe battery 924 and the electrically conductive housing 924 can be via awire or other electrical conductor. Alternatively, the battery 924 canbe designed and manufactured such that the outer-casing of the battery924 is electrically connected to the negative (−) pole of the battery,or more generally, provides the negative (−) pole for the battery. Wherethe outer-casing of the battery 924 provides the negative (−) pole ofthe battery, then the negative (−) pole of the battery 924 will beconnected to the electrically conductive housing 922 so long as theouter-casing of the battery 924 is physically in contact with theelectrically conductive housing 922. Unless stated otherwise, it will beassumed that the outer-casing of the battery 924 provides the negative(−) pole of the battery. However, it should be noted that in alternativeembodiments the battery 924 can be designed and manufactured such thatthe outer-casing of the battery 924 is electrically connected to thepositive (+) pole of the battery, or more generally, provides thepositive (+) pole for the battery.

Conductors 934 and 936 that extend through the inter-housing insulator932 connect the positive (+) and negative (−) poles of the battery 924to the electronics 914, which are encased within the electricallyconductive housing 912, to thereby enable the battery 924 to providepower to the electronics 914.

The LP 902 a is shown as including a tip electrode 908 that is locatedadjacent to the free end of the housing 912. The tip electrode 908 iselectrically isolated from the electrically conductive housing 912 by aninsulator 906. A feedthrough 908 that extends through the insulator 906is used to connect the tip electrode 908 to the electronics 914 (e.g.,one or more pulse generators and/or one or more sense amplifiers). Thetip electrode 908 can have various different shapes, depending uponimplementation. For example, the tip electrode 908 can have an annularshape, a semi-spherical cap, or a helical shape to enable the tipelectrode 908 to also function as an attachment mechanism for attachingthe LP 902 to an interior or exterior wall of a cardiac chamber. Wherethe tip electrode 908 has a helical shape it can also be referred to asa helical or helix electrode. Other shapes for the tip electrode 908 arealso possible and within the embodiments of the present technologydescribed herein.

The LP 902 a is also shown as including a ring electrode 918 and a ringelectrode 928. In certain embodiments, the ring electrode 918 isprovided by a non-insulated portion of the electrically conductivehousing 912. More specifically, portions of the electrically conductivehousing 912 can be coated or otherwise covered by an insulator 916, anda non-insulated portion of the housing 912 can provide the ringelectrode 918. Similarly, the ring electrode 928 can be provided by anon-insulated portion of the electrically conductive housing 922. Morespecifically, portions of the electrically conductive housing 922 can becoated or otherwise covered by an insulator 926, and a non-insulatedportion of the housing 922 can provide the ring electrode 928. Suchinsulators 916 and 926 can be made various different types ofbiocompatible insulating materials, such as, but not limited to,ceramic, polyurethane, parylene, or silicone.

Where the outer-casing of the battery 924 provides the negative (−) poleof the battery, and the outer-casing of the battery 924 is physically incontact with the electrically conductive housing 922, then theelectrically conductive housing 922 is electrically connected to thenegative (−) pole (aka the negative terminal) of the battery 924. Whenusing such a battery 924, an advantage of having the inter-housinginsulator 932 electrically isolate the housings 912 and 922 from oneanother is that a single common feedthrough (936 in FIG. 9A) can be usedto connect the electronics 914 to the negative (−) pole of the battery924 and the ring electrode 928. A further advantage of having theinter-housing insulator 932 electrically isolate the housings 912 and922 from one another is that a non-insulated portion of each of thehousings can be used to provide respective ring electrodes 918 and 928.Another advantage of having the inter-housing insulator 932 electricallyisolate the housings 912 and 922 from one another is that the ringelectrodes 918 and 928 can be used independently of one another.Further, it is noted that designing an LP to include three electrodesshould enable better sensing of far-field signals, compared to if the LPincluded only two electrodes.

In accordance with certain embodiments, during pacing of a cardiacchamber (e.g., RA chamber or RV chamber) within or on which the LP 902 ais implanted, the tip electrode 908 is connected as the cathode and oneof the ring electrodes 918 or 928 is connected as the anode. In otherwords, a tip-to-ring pacing vector can be used for pacing. It would alsobe possible that when performing pacing using the tip electrode 908 asthe cathode, both ring electrodes 918 and 928 can be connected as theanode (e.g., a distributed anode) at the same time. In accordance withcertain embodiments, sensing can be performed using one or more pair ofthe electrodes 902, 918, and 928. Additionally, i2i communication can beperformed using a pair of the electrodes 902, 918, and 928.

In an embodiment, if the LP 902 a is implanted within the RV chamber,then near-field sensing (of electrical cardiac activity associated withthe RV chamber) can be performed using the tip electrode 908 and thering electrode 918. In other words, a tip-to-ring sensing vector can beused for near-field sensing. In an embodiment, if the LP 902 a isimplanted in the RV chamber, then far-field sensing (of electricalcardiac activity associated with the RA chamber) can be performed usingthe tip electrode 908 and the ring electrode 928, since the ringelectrode 928 will be the electrode closest to the RA chamber. In otherwords, a separate tip-to-ring sensing vector can be used for far-fieldsensing than is used for near-field sensing. In still anotherembodiment, if the LP 902 a is implanted in the RV chamber, far-fieldsensing (of electrical cardiac activity associated with the RA chamber)can be performed using the ring electrode 918 and the ring electrode928. In other words, a ring-to-ring sensing vector can be used forfar-field sensing. The tip electrode 908 and the ring electrode 928 canbe used for i2i communication (both transmitting of i2i pulses andreceiving of i2i pulses) with another LP, another IMD, and/or with anexternal device (e.g., programmer). Alternatively, the tip electrode 908and the ring electrode 918 can be used for i2i communication (bothtransmitting of i2i pulses and receiving of i2i pulses) with another LP,another IMD, and/or with an external device. In still other embodiments,the ring electrode 918 and the ring electrode 928 can be used for i2icommunication (both transmitting of i2i pulses and receiving of i2ipulses) with another LP, another IMD, and/or with an external device.

In an embodiment, if the LP 902 a is implanted within the RA chamber,then near-field sensing (of electrical cardiac activity associated withthe RA chamber) can be performed using the tip electrode 908 and thering electrode 918 (i.e., a tip-to-ring sensing vector can be used fornear-field sensing); far-field sensing (of electrical cardiac activityassociated with the RV chamber) can be performed using the tip electrode908 and the ring electrode 928 (since the ring electrode 928 will be theelectrode closest to the RV chamber) or using the ring electrode 918 andthe ring electrode 928; and the tip electrode 908 and one of the ringelectrodes 918 or 928 can be used for i2i communication (bothtransmitting of i2i pulses and receiving of i2i pulses) with another LP,another IMD, and/or with an external device, or the two ring electrode918 and 928 can be used for i2i communication.

More generally, when the LP 902 a is implanted within a cardiac chamber,near-field sensing (of electrical cardiac activity associated with thelocal chamber within which the LP 902 a is implanted) can be performedusing the tip electrode 908 and the ring electrode 918 (i.e., atip-to-ring sensing vector can be used for near-field sensing);far-field sensing (of electrical cardiac activity associated with aremote chamber) can be performed using the tip electrode 908 and thering electrode 928, or using the ring electrode 918 and the ringelectrode 928; and i2i communication can be performed using the tipelectrode 908 and one of the ring electrodes 918 or 928, or using bothring electrode 918 and 928. Other variations are also possible andwithin the scope of the embodiments of the present technology.

Still referring to FIG. 9A, the LP 902 a is also shown as including aretrieval feature 942, which can include a “button” or circular graspingfeature that is configured to dock within a docking cap or a retrievalcatheter that can be used to remove the LP 902 a when it needs to beremoved and/or replaced. Alternative form factors for the retrievalfeature are also possible. The retrieval feature 942 can be made of anon-electrically conductive material, i.e., an insulating material.Alternatively, the retrieval feature 942 can be made of an electricallyconductive material, and in the embodiment of FIG. 9A, can be coated orotherwise covered by a biocompatible insulating materials, such as, butnot limited to, ceramic, polyurethane, parylene, or silicone.

FIG. 9B is an illustration of a leadless pacemaker (LP) 902 b accordingto another embodiment of the present technology. Elements in FIG. 9Bthat are the same or similar to elements in FIG. 9A are numbered thesame and need not be described again in the same amount of detail. Inaccordance with certain embodiments, the retrieval feature 942 (or atleast a portion thereof) is made of an electrically conductive materialand is electrically connected to the negative (−) pole of the battery924. This enables the retrieval feature 942 (or at least a portionthereof) to be another tip electrode 948. Accordingly, the LP 902 bincludes the tip electrode 908 adjacent to the free end of the housing912, as well as a tip electrode adjacent to the free end of the housing922. Where the retrieval feature 942 provides a tip electrode 948, itcan also be referred to as the tip electrode 942/948. A comparisonbetween FIGS. 9A and 9B shows that a distinction of the LP 902 b is thatinstead of having one tip electrode and two ring electrodes, as was thecase with the LP 902 a, the LP 902 b has two tip electrodes and one ringelectrode. It would also be possible that a portion of the free end ofthe housing 922 is not coated or otherwise covered with an insulatedmaterial to thereby provide a tip electrode adjacent to the free end ofthe housing 922. This would also provide the LP 902 b with two tipelectrodes and one ring electrode.

In accordance with certain embodiments, during pacing of a cardiacchamber (e.g., RA chamber or RV chamber) within or on which the LP 902 bis implanted, the tip electrode 908 is connected as the cathode and thering electrodes 918 is connected as the anode. In other words, atip-to-ring pacing vector can be used for pacing. In accordance withcertain embodiments, sensing can be performed using one or more pair ofthe electrodes 902, 918, and 948. Additionally, i2i communication can beperformed using a pair of the electrodes 902, 918, and 948.

In an embodiment, if the LP 902 b is implanted within the RV chamber,then near-field sensing (of electrical cardiac activity associated withthe RV chamber) can be performed using the tip electrode 908 and thering electrode 918. In other words, a tip-to-ring sensing vector can beused for near-field sensing. In an embodiment, if the LP 902 b isimplanted in the RV chamber, then far-field sensing (of electricalcardiac activity associated with the RA chamber) can be performed usingthe tip electrode 908 and the tip electrode 948, since the tip electrode948 will be the electrode closest to the RA chamber. In other words, atip-to-tip sensing vector can be used for far-field sensing. In stillanother embodiment, if the LP 902 b is implanted in the RV chamber,far-field sensing (of electrical cardiac activity associated with the RAchamber) can be performed using the ring electrode 918 and the tipelectrode 948. In other words, a ring-to-tip sensing vector can be usedfor far-field sensing. The tip electrode 908 and the tip electrode 948can be used for i2i communication (both transmitting of i2i pulses andreceiving of i2i pulses) with another LP, another IMD, and/or forimplant to programmer (i2p) communication with an external device.Alternatively, the ring electrode 918 and the tip electrode 948 can beused for i2i communication (both transmitting of i2i pulses andreceiving of i2i pulses) with another LP, another IMD, and/or for i2pcommunication with an external device.

In an embodiment, if the LP 902 b is implanted within the RA chamber,then near-field sensing (of electrical cardiac activity associated withthe RA chamber) can be performed using the tip electrode 908 and thering electrode 918 (i.e., a tip-to-ring sensing vector can be used fornear-field sensing); far-field sensing (of electrical cardiac activityassociated with the RV chamber) can be performed using the tip electrode908 and the tip electrode 948 (since the tip electrode 948 will be theelectrode closest to the RV chamber) or using the ring electrode 918 andthe tip electrode 948. The tip electrode 908 and the tip electrode 948,or the ring electrode 918 and the tip electrode 948, can be used for i2icommunication (both transmitting of i2i pulses and receiving of i2ipulses) with another LP, another IMD, and/or for i2p communication withan external device.

More generally, when the LP 902 b is implanted within a cardiac chamber,near-field sensing (of electrical cardiac activity associated with thelocal chamber within which the LP 902 b is implanted) can be performedusing the tip electrode 908 and the ring electrode 918 (i.e., atip-to-ring sensing vector can be used for near-field sensing);far-field sensing (of electrical cardiac activity associated with aremote chamber) can be performed using the tip electrode 908 and the tipelectrode 948, or using the ring electrode 918 and the tip electrode948; and i2i communication can be performed using the tip electrode 908and the tip electrode 948, or using the ring electrode 918 and the tipelectrode 948. Other variations are also possible and within the scopeof the embodiments of the present technology.

Referring briefly back to FIG. 1B, only one sense amplifier 132 wasshown within the LP illustrated therein. In accordance with certainembodiments, an LP (e.g., 102, 104, 902 a or 902 b) includes multiplesense amplifier, e.g., one or more for sensing near-field signals, oneor more for sensing far-field signals, and one or more for sensing i2isignals. In FIG. 1B only one pulse generator 116 was shown within the LPillustrated therein. In accordance with certain embodiments, an LP(e.g., 102, 104, 902 a or 902 b) includes multiple pulse generators,e.g., one for generating pacing signals, and one or more for generatingi2i signals. Further, it should be noted that where an LP (e.g., 102,104, 902 a or 902 b) includes three or more electrodes, switch circuitrycan be located between the electrodes and the sense amplifier(s) andpulse generator(s), to enable a controller to control which electrodesare used to sense a near-field signal, which electrodes are used tosense a far-field signal, which electrodes are used for pacing, and tocontrol which electrodes are used for i2i communications.

The high level flow diagram in FIG. 10A will now be used to summarizecertain embodiments of the present technology that can be used with theLP 902 a described above with reference to FIG. 9A. Referring brieflyback to FIG. 9A, the LP 902 a was shown as including three electrodes,including a tip electrode 908, a first ring electrode 918, and a secondring electrode 928. The electronic circuitry 914 within the housing 912can include one or more pulse generators, one or more sense amplifiers,and a controller. The battery 924 within the housing 922 provides powerto the electronic circuitry via conductors that electrically couplepositive and negative poles of the battery 924 within the housing 922 tothe electronic circuitry 914 within the housing 912.

Referring to FIG. 10A, step 1002 involves using a sensing vectorincluding the tip electrode 908 and the ring electrode 918 to sense anear-field signal indicative of electrical cardiac activity associatedwith the local cardiac chamber of the patient's heart in or on which theLP is implanted. Step 1004 involves using a sensing vector including thering electrodes 918 and 928 to sense a far-field signal indicative ofelectrical cardiac activity associated with a remote cardiac chamber ofthe patient's heart in or on which the LP 902 a is not implanted. Step1006 involves using a pacing vector including the tip electrode 908 andthe ring electrode 918 to deliver pacing pulses to the local cardiacchamber in or on which the LP 902 a is implanted. Step 1006 can alsoinvolve timing delivery of the pacing pulses based on the near-fieldsignal (sensed at step 1002) and/or based on the far-field signal(sensed at step 1004). Step 1008 involves using the ring electrodes 918and 928 (or the tip electrode 908 and the ring electrode 928) totransmit and/or receive communication pulses from another implantablemedical device or an external device. It would also be possible to timedelivery of pacing pulses (delivered at step 1006) based on i2i messagesreceived from another implantable medical device (e.g., another LP).Depending upon the specific implementation, the order of the varioussteps shown in FIG. 10A can be rearranged, and thus, embodiments are notintended to be limited to the order shown in FIG. 10A. It would also bepossible that just subsets of the steps shown in FIG. 10A be performed.Other variations of the methods summarized with reference to FIG. 10Acould be appreciated from the above discussion of FIG. 9A.

The high level flow diagram in FIG. 10B will now be used to summarizecertain embodiments of the present technology that can be used with theLP 902 b described above with reference to FIG. 9B. Referring brieflyback to FIG. 9A, the LP 902 b was shown as including three electrodes,including a first tip electrode 908, a ring electrode 918, and a secondtip electrode 948. Again, the electronic circuitry 914 within thehousing 912 can include one or more pulse generators, one or more senseamplifiers, and a controller, and the battery 924 within the housing 922provides power to the electronic circuitry via conductors thatelectrically couple positive and negative poles of the battery 924within the housing 922 to the electronic circuitry 914 within thehousing 912.

Referring to FIG. 10B, step 1012 involves using a sensing vectorincluding the tip electrode 908 and the ring electrode 918 to sense anear-field signal indicative of electrical cardiac activity associatedwith the local cardiac chamber of the patient's heart in or on which theLP 902 b is implanted. Step 1014 involves using a sensing vectorincluding the ring electrode 918 and the tip electrode 948 to sense afar-field signal indicative of electrical cardiac activity associatedwith a remote cardiac chamber of the patient's heart in or on which theLP 902 b is not implanted. Step 1016 involves using a pacing vectorincluding the tip electrode 908 and the ring electrode 918 to deliverpacing pulses to the local cardiac chamber in or on which the LP 902 bis implanted. Step 1016 can also involve timing delivery of the pacingpulses based on the near-field signal (sensed at step 1012) and/or basedon the far-field signal (sensed at step 1014). Step 1018 involves usingthe ring electrode 918 and the tip electrode 948 (or the tip electrode908 and the tip electrode 948) to transmit and/or receive communicationpulses from another implantable medical device or an external device. Itwould also be possible to time delivery of pacing pulses (delivered atstep 1016) based on i2i messages received from another implantablemedical device (e.g., another LP). Depending upon the specificimplementation, the order of the various steps shown in FIG. 10B can berearranged, and thus, embodiments are not intended to be limited to theorder shown in FIG. 10B. It would also be possible that just subsets ofthe steps shown in FIG. 10B be performed. Other variations of themethods summarized with reference to FIG. 10B could be appreciated fromthe above discussion of FIG. 9B. For example, the second tip electrode948, instead of being provided by the retrieval feature 942 (or aportion thereof), can be provided by a portion of the free end of thehousing 922 that is not covered by the insulator 926.

FIG. 11 shows a block diagram of showing exemplary further details of anLP 1101 (e.g., 102, 104, 902 a, or 902 b) that is implanted into thepatient as part of the implantable cardiac system in accordance withcertain embodiments herein. LP 1101 may be implemented as afull-function biventricular pacemaker, equipped with both atrial andventricular sensing and pacing circuitry for four chamber sensing andstimulation therapy (including both pacing and shock treatment).Optionally, LP 1101 may provide full-function cardiac resynchronizationtherapy. Alternatively, LP 1101 may be implemented with a reduced set offunctions and components. For instance, the LP may be implementedwithout ventricular sensing and pacing.

LP 1101 has a housing 1100 to hold the electronic/computing components.Housing 1100 (which is often referred to as the “can”, “case”,“encasing”, or “case electrode”) may be programmably selected to act asthe return electrode for certain stimulus modes. Housing 1100 mayfurther include a connector (not shown) with a plurality of terminals1102, 1104, 1106, 1108, and 1110. The terminals may be connected toelectrodes that are located in various locations on housing 1100 orelsewhere within and about the heart. LP 1101 includes a programmablemicrocontroller 1120 that controls various operations of LP 1101,including cardiac monitoring and stimulation therapy. Microcontroller1120 includes a microprocessor (or equivalent control circuitry), RAMand/or ROM memory, logic and timing circuitry, state machine circuitry,and I/O circuitry. The microcontroller is an example of a controller(e.g., 112) discussed above.

LP 1101 further includes a pulse generator 1122 that generatesstimulation pulses and communication pulses for delivery by one or moreelectrodes coupled thereto. Pulse generator 1122 is controlled bymicrocontroller 1120 via control signal 1124. Pulse generator 1122 maybe coupled to the select electrode(s) via an electrode configurationswitch 1126, which includes multiple switches for connecting the desiredelectrodes to the appropriate I/O circuits, thereby facilitatingelectrode programmability. Switch 1126 is controlled by a control signal1128 from microcontroller 1120.

In FIG. 11, a single pulse generator 1122 is illustrated. Optionally,the LP may include multiple pulse generators, similar to pulse generator1122, where each pulse generator is coupled to one or more electrodesand controlled by microcontroller 1120 to deliver select stimuluspulse(s) to the corresponding one or more electrodes. For example, onepulse generator can be used to generate pacing pulses, and another pulsegenerator can be used to generate i2i pulses.

Microcontroller 1120 is illustrated as including timing controlcircuitry 1132 to control the timing of the stimulation pulses (e.g.,pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A)delay, or ventricular interconduction (V-V) delay, etc.). Timing controlcircuitry 1132 may also be used for the timing of refractory periods,blanking intervals, noise detection windows, evoked response windows,alert intervals, marker channel timing, and so on. Microcontroller 1120may also have an arrhythmia detector 1134 for detecting arrhythmiaconditions and a morphology detector 1136. Although not shown, themicrocontroller 1120 may further include other dedicated circuitryand/or firmware/software components that assist in monitoring variousconditions of the patient's heart and managing pacing therapies. Themicrocontroller can include a processor. The microcontroller, and/or theprocessor thereof, can be used to perform the methods of the presenttechnology described herein.

LP 1101 is further equipped with a communication modem(modulator/demodulator) 1140 to enable wireless communication with theremote slave pacing unit. Modem 1140 may include one or moretransmitters and one or more receivers as discussed herein in connectionwith FIG. 1B. In one implementation, modem 1140 may use low or highfrequency modulation. As one example, modem 1140 may transmit i2imessages and other signals through conductive communication between apair of electrodes. Modem 1140 may be implemented in hardware as part ofmicrocontroller 1120, or as software/firmware instructions programmedinto and executed by microcontroller 1120. Alternatively, modem 1140 mayreside separately from the microcontroller as a standalone component.

LP 1101 includes a sensing circuit 1144 selectively coupled to one ormore electrodes, that perform sensing operations, through switch 1126 todetect the presence of cardiac activity associated with one or morechambers of the heart. Sensing circuit 1144 may include dedicated senseamplifiers, multiplexed amplifiers, or shared amplifiers. It may furtheremploy one or more low power, precision amplifiers with programmablegain and/or automatic gain control, bandpass filtering, and thresholddetection circuit to selectively sense the cardiac signal of interest.The automatic gain control enables the unit to sense low amplitudesignal characteristics of atrial fibrillation. Switch 1126 determinesthe sensing polarity of the cardiac signal by selectively closing theappropriate switches. In this way, the clinician may program the sensingpolarity independent of the stimulation polarity.

The output of sensing circuit 1144 is connected to microcontroller 1120which, in turn, triggers or inhibits the pulse generator 1122 inresponse to the presence or absence of cardiac activity. Sensing circuit1144 receives a control signal 1146 from microcontroller 1120 forpurposes of controlling the gain, threshold, polarization charge removalcircuitry (not shown), and the timing of any blocking circuitry (notshown) coupled to the inputs of the sensing circuitry.

In FIG. 11, a single sensing circuit 1144 is illustrated. Optionally,the LP may include multiple sensing circuits, similar to sensing circuit1144, where each sensing circuit is coupled to one or more electrodesand controlled by microcontroller 1120 to sense electrical activitydetected at the corresponding one or more electrodes. For example, onesensing circuit can be used to sense near-field signals, another sensingcircuit can be used to sense far-field signals, and one or more furthersensing circuits can be used to sense i2i signals.

LP 1101 further includes an analog-to-digital (ND) data acquisitionsystem (DAS) 1150 coupled to one or more electrodes via switch 1126 tosample cardiac signals across any pair of desired electrodes. Dataacquisition system 1150 is configured to acquire intracardiacelectrogram signals, convert the raw analog data into digital data, andstore the digital data for later processing and/or telemetrictransmission to an external device 1154 (e.g., a programmer, localtransceiver, or a diagnostic system analyzer). Data acquisition system1150 is controlled by a control signal 1156 from the microcontroller1120.

Microcontroller 1120 is coupled to a memory 1160 by a suitabledata/address bus. The programmable operating parameters used bymicrocontroller 1120 are stored in memory 1160 and used to customize theoperation of LP 1101 to suit the needs of a particular patient. Suchoperating parameters define, for example, pacing pulse amplitude, pulseduration, electrode polarity, rate, sensitivity, automatic features,arrhythmia detection criteria, and the amplitude, waveshape and vectorof each shocking pulse to be delivered to the patient's heart withineach respective tier of therapy.

The operating parameters of LP 1101 may be non-invasively programmedinto memory 1160 through a telemetry circuit 1164 in telemetriccommunication via communication link 1166 with external device 1154.Telemetry circuit 1164 allows intracardiac electrograms and statusinformation relating to the operation of LP 1101 (as contained inmicrocontroller 1120 or memory 1160) to be sent to external device 1154through communication link 1166.

LP 1101 can further include magnet detection circuitry (not shown),coupled to microcontroller 1120, to detect when a magnet is placed overthe unit. A magnet may be used by a clinician to perform various testfunctions of LP 1101 and/or to signal microcontroller 1120 that externaldevice 1154 is in place to receive or transmit data to microcontroller1120 through telemetry circuits 1164.

LP 1101 can further include one or more physiological sensors 1170. Suchsensors are commonly referred to as “rate-responsive” sensors becausethey are typically used to adjust pacing stimulation rates according tothe exercise state of the patient. However, physiological sensor 1170may further be used to detect changes in cardiac output, changes in thephysiological condition of the heart, or diurnal changes in activity(e.g., detecting sleep and wake states). Signals generated byphysiological sensors 1170 are passed to microcontroller 1120 foranalysis. Microcontroller 1120 responds by adjusting the various pacingparameters (such as rate, AV Delay, V-V Delay, etc.) at which the atrialand ventricular pacing pulses are administered. While shown as beingincluded within LP 1101, physiological sensor(s) 1170 may be external toLP 1101, yet still be implanted within or carried by the patient.Examples of physiologic sensors might include sensors that, for example,sense temperature, respiration rate, pH of blood, ventricular gradient,activity, position/posture, minute ventilation (MV), and so forth. Thephysiological sensors 1170 can include, e.g., an accelerometer (e.g.,154 in FIG. 1B) and/or a pressure sensor (e.g., 156 in FIG. 1B).

A battery 1172 provides operating power to all of the components in LP1101. Battery 1172 is preferably capable of operating at low currentdrains for long periods of time. Battery 1172 also desirably has apredictable discharge characteristic so that elective replacement timecan be detected. As one example, LP 1101 employs a lithium carbonmonofluoride (Li-CFx) battery. In certain embodiments, examples of whichwere described above with reference to FIGS. 9A and 9B, the battery 1172(which was labeled 924 in FIGS. 9A and 9B) can be located in a firsthermetic electrically conductive housing, and the microcontroller 1120and other circuitry can be located in a second hermetic electricallyconductive housing.

LP 1101 further includes an impedance measuring circuit 1174, which canbe used for many things, including: lead impedance surveillance duringthe acute and chronic phases for proper lead positioning ordislodgement; detecting operable electrodes and automatically switchingto an operable pair if dislodgement occurs; measuring respiration orminute ventilation; measuring thoracic impedance for determining shockthresholds; detecting when the device has been implanted; measuringstroke volume; and detecting the opening of heart valves; and so forth.Impedance measuring circuit 1174 is coupled to switch 1126 so that anydesired electrode may be used. In this embodiment LP 1101 furtherincludes a shocking circuit 1180 coupled to microcontroller 1120 by adata/address bus 1182.

In some embodiments, an LP is configured to be implantable in anychamber of the heart, namely either atrium (RA, LA) or either ventricle(RV, LV). Furthermore, for dual-chamber configurations, multiple LPs maybe co-implanted (e.g., one in the RA and one in the RV, one in the RVand one in the coronary sinus proximate the LV). Certain pacemakerparameters and functions depend on (or assume) knowledge of the chamberin which the pacemaker is implanted (and thus with which the LP isinteracting; e.g., pacing and/or sensing). Some non-limiting examplesinclude: sensing sensitivity, an evoked response algorithm, use of AFsuppression in a local chamber, blanking & refractory periods, etc.Accordingly, each LP needs to know an identity of the chamber in whichthe LP is implanted, and processes may be implemented to automaticallyidentify a local chamber associated with each LP.

Processes for chamber identification may also be applied to subcutaneouspacemakers, ICDs, with leads and the like. A device with one or moreimplanted leads, identification and/or confirmation of the chamber intowhich the lead was implanted could be useful in several pertinentscenarios. For example, for a DR or CRT device, automatic identificationand confirmation could mitigate against the possibility of the clinicianinadvertently placing the V lead into the A port of the implantablemedical device, and vice-versa. As another example, for an SR device,automatic identification of implanted chamber could enable the deviceand/or programmer to select and present the proper subset of pacingmodes (e.g., AAI or WI), and for the IPG to utilize the proper set ofsettings and algorithms (e.g., V-AutoCapture vs ACap-Confirm, sensingsensitivities, etc.).

It is to be understood that the subject matter described herein is notlimited in its application to the details of construction and thearrangement of components set forth in the description herein orillustrated in the drawings hereof. The subject matter described hereinis capable of other embodiments and of being practiced or of beingcarried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Further, it is noted that the term “basedon” as used herein, unless stated otherwise, should be interpreted asmeaning based at least in part on, meaning there can be one or moreadditional factors upon which a decision or the like is made. Forexample, if a decision is based on the results of a comparison, thatdecision can also be based on one or more other factors in addition tobeing based on results of the comparison.

Embodiments of the present technology have been described above with theaid of functional building blocks illustrating the performance ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have often been defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed. Any such alternate boundaries are thus withinthe scope and spirit of the claimed invention. For example, it would bepossible to combine or separate some of the steps shown in FIGS. 6, 7,8, 10A and 10B. For another example, it is possible to change theboundaries of some of the dashed blocks shown in FIGS. 1B and 11.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the embodiments ofthe present technology without departing from its scope. While thedimensions, types of materials and coatings described herein areintended to define the parameters of the embodiments of the presenttechnology, they are by no means limiting and are exemplary embodiments.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the embodiments ofthe present technology should, therefore, be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. In the appended claims, the terms“including” and “in which” are used as the plain-English equivalents ofthe respective terms “comprising” and “wherein.” Moreover, in thefollowing claims, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements on their objects. Further, the limitations of the followingclaims are not written in means—plus-function format and are notintended to be interpreted based on 35 U.S.C. § 112(f), unless and untilsuch claim limitations expressly use the phrase “means for” followed bya statement of function void of further structure.

What is claimed is:
 1. An implantable system, comprising: a firstleadless pacemaker (LP1) configured to be implanted in or on a firstchamber of a heart; and a second leadless pacemaker (LP2) configured tobe implanted in or on a second chamber of the heart; the LP1 configuredto time delivery of one or more pacing pulses delivered to the firstchamber of the heart based on timing of cardiac activity associated withthe second chamber of the heart detected by the LP1 itself; the LP1configured to transmit implant-to-implant (i2i) messages to the LP2; andthe LP2 configured to time delivery of one or more pacing pulsesdelivered to the second chamber of the heart based on timing of cardiacactivity associated with the second chamber of the heart as determinedbased on one or more i2i messages received by the LP2 from the LP1. 2.The implantable system of claim 1, wherein: the LP1 is configured to beimplanted in or on a right ventricular (RV) chamber; and the LP2 isconfigured to be implanted in or on a right atrial (RA) chamber.
 3. Theimplantable system of claim 1, wherein: the LP1 is configured to beimplanted in or on a right atrial (RA) chamber; and the LP2 isconfigured to be implanted in or on a right ventricular (RV) chamber. 4.The implantable system of claim 1, wherein the LP1 is configured to:obtain a far-field signal from which electrical cardiac activityassociated with the second chamber of the heart may be detected; and usethe far-field signal to thereby time delivery of one or more pacingpulses delivered to the first chamber of the heart, based on timing ofcardiac activity associated with the second chamber of the heartdetected by the LP1 itself from the far-field signal.
 5. The implantablesystem of claim 4, wherein: the LP1 is configured to be implanted in oron a right ventricular (RV) chamber; and the LP2 is configured to beimplanted in or on a right atrial (RA) chamber.
 6. The implantablesystem of claim 4, wherein: the LP1 is configured to be implanted in oron a right atrial (RA) chamber; and the LP2 is configured to beimplanted in or on a right ventricular (RV) chamber.
 7. The implantablesystem of claim 1, wherein the LP1 is configured to: obtain a sensorsignal from which mechanical cardiac activity associated with the secondchamber of the heart may be detected; and use the sensor signal tothereby time delivery of one or more pacing pulses delivered to thefirst chamber of the heart, based on timing of cardiac activityassociated with the second chamber of the heart detected by the LP1itself from the sensor signal.
 8. The implantable system of claim 7,wherein: the LP1 is configured to be implanted in or on a rightventricular (RV) chamber; and the LP2 is configured to be implanted inor on a right atrial (RA) chamber.
 9. The implantable system of claim 7,wherein: the LP1 is configured to be implanted in or on a right atrial(RA) chamber; and the LP2 is configured to be implanted in or on a rightventricular (RV) chamber.
 10. The implantable system of claim 1,wherein: the LP1 is configured to be implanted in or on a right atrial(RA) chamber; and the LP2 is configured to be implanted in or on a rightventricular (RV) chamber; the LP1 is configured to perform at least oneof AAI, ADI or ADD pacing; the LP2 is configured to perform at least oneof WI, VDI or VDD pacing; and the LP1 and the LP2 collectively providingDDD or DDI pacing or some other dual chamber pacing mode that providessynchronization between the LP1 and the LP2.
 11. The implantable systemof claim 1, wherein: the LP1 is configured to be implanted in or on aright ventricular (RV) chamber; and the LP2 is configured to beimplanted in or on a right atrial (RA) chamber; the LP1 is configured toperform at least one of WI, VDI or VDD pacing; and the LP2 is configuredto perform at least one of AAI, ADI or ADD pacing; the LP1 and the LP2collectively providing DDD or DDI pacing or some other dual chamberpacing mode that provides synchronization between the LP1 and the LP2.12. A method for use with an implantable system including a firstleadless pacemaker (LP1) configured to be implanted in or on a firstchamber of a heart, and a second leadless pacemaker (LP2) configured tobe implanted in or on a second chamber of the heart, the methodcomprising: the LP1 timing delivery of one or more pacing pulsesdelivered to the first chamber of the heart based on timing of cardiacactivity associated with the second chamber of the heart detected by theLP1 itself; the LP1 transmitting implant-to-implant (i2i) messages tothe LP2; and the LP2 timing delivery of one or more pacing pulsesdelivered to the second chamber of the heart based on timing of cardiacactivity associated with the second chamber of the heart as determinedbased on one or more i2i messages received by the LP2 from the LP1. 13.The method of claim 12, wherein the LP1 is configured to be implanted inor on a right ventricular (RV) chamber, the LP2 is configured to beimplanted in or on a right atrial (RA) chamber, and wherein: the LP1timing delivery of one or more pacing pulses delivered to the firstchamber of the heart comprises the LP1 timing delivery of one or morepacing pulses delivered to the RV chamber based on timing of cardiacactivity associated with the RA chamber detected by the LP1 itself; andthe LP2 timing delivery of one or more pacing pulses delivered to thesecond chamber of the heart comprises the LP2 timing delivery of one ormore pacing pulses delivered to the RA chamber based on timing ofcardiac activity associated with the RV chamber as determined based onone or more i2i messages received by the LP2 from the LP1.
 14. Themethod of claim 12, wherein the LP1 is configured to be implanted in oron a right atrial (RA) chamber, the LP2 is configured to be implanted inor on a right ventricular (RV) chamber, and wherein: the LP1 timingdelivery of one or more pacing pulses delivered to the first chamber ofthe heart comprises the LP1 timing delivery of one or more pacing pulsesdelivered to the RA chamber based on timing of cardiac activityassociated with the RV chamber detected by the LP1 itself; and the LP2timing delivery of one or more pacing pulses delivered to the secondchamber of the heart comprises the LP2 timing delivery of one or morepacing pulses delivered to the RV chamber based on timing of cardiacactivity associated with the RA chamber as determined based on one ormore i2i messages received by the LP2 from the LP1.
 15. The method ofclaim 12, wherein the LP1 timing delivery of one or more pacing pulsesdelivered to the first chamber of the heart comprises: the LP1 obtaininga far-field signal from which electrical cardiac activity associatedwith the second chamber of the heart may be detected; and the LP1 usingthe far-field signal to thereby time delivery of one or more pacingpulses delivered to the first chamber of the heart, based on timing ofcardiac activity associated with the second chamber of the heartdetected by the LP1 itself from the far-field signal.
 16. The method ofclaim 15, wherein the LP1 is configured to be implanted in or on a rightventricular (RV) chamber, the LP2 is configured to be implanted in or ona right atrial (RA) chamber, and wherein: the LP1 obtaining a far-fieldsignal comprises the LP1 obtaining a far-field signal from whichelectrical cardiac activity associated with the RA chamber may bedetected; and the LP1 using the far-field signal comprises the LP1 usingthe far-field signal to thereby time delivery of one or more pacingpulses delivered to the RV chamber, based on timing of cardiac activityassociated with the RA chamber detected by the LP1 itself from thefar-field signal.
 17. The method of claim 15, wherein the LP1 isconfigured to be implanted in or on a right atrial (RA) chamber, the LP2is configured to be implanted in or on a right ventricular (RV) chamber,and wherein: the LP1 obtaining a far-field signal comprises the LP1obtaining a far-field signal from which electrical cardiac activityassociated with the RV chamber may be detected; and the LP1 using thefar-field signal comprises the LP1 using the far-field signal to therebytime delivery of one or more pacing pulses delivered to the RA chamber,based on timing of cardiac activity associated with the RV chamberdetected by the LP1 itself from the far-field signal.
 18. The method ofclaim 13, wherein the LP1 timing delivery of one or more pacing pulsesdelivered to the first chamber of the heart comprises: the LP1 obtaininga sensor signal from which mechanical cardiac activity associated withthe second chamber of the heart may be detected; and the LP1 using thesensor signal to thereby time delivery of one or more pacing pulsesdelivered to the first chamber of the heart, based on timing of cardiacactivity associated with the second chamber of the heart detected by theLP1 itself from the sensor signal.
 19. The method of claim 18, whereinthe LP1 is configured to be implanted in or on a right ventricular (RV)chamber, the LP2 is configured to be implanted in or on a right atrial(RA) chamber, and wherein: the LP1 obtaining a sensor signal comprisesthe LP1 obtaining a sensor signal from which mechanical cardiac activityassociated with the RA chamber may be detected; and the LP1 using thesensor signal comprises the LP1 using the sensor signal to thereby timedelivery of one or more pacing pulses delivered to the RV chamber, basedon timing of cardiac activity associated with the RA chamber detected bythe LP1 itself from the sensor signal.
 20. The method of claim 18,wherein the LP1 is configured to be implanted in or on a right atrial(RA) chamber, the LP2 is configured to be implanted in or on a rightventricular (RV) chamber, and wherein: the LP1 obtaining a sensor signalcomprises the LP1 obtaining a sensor signal from which mechanicalcardiac activity associated with the RV chamber may be detected; and theLP1 using the sensor signal comprises the LP1 using the sensor signal tothereby time delivery of one or more pacing pulses delivered to the RAchamber, based on timing of cardiac activity associated with the RVchamber detected by the LP1 itself from the sensor signal.
 21. Animplantable system, comprising: a first leadless pacemaker (LP1)configured to be implanted in or on a right atrial (RA) chamber; and asecond leadless pacemaker (LP2) configured to be implanted in or on aright ventricular (RV) chamber; the LP1 configured to perform at leastone of AAI, ADI or ADD pacing; the LP2 configured to perform at leastone of VVI, VDI or VDD pacing; and the LP1 and the LP2 collectivelyproviding DDD or DDI pacing or some other dual chamber pacing mode thatprovides synchronization between the LP1 and the LP2.